f2fs: fix wrong dirty page count when race between mmap and fallocate.

f2fs: fix wrong dirty page count when race between mmap and fallocate.

This is a BUG_ON issue as follows when running xfstest-generic-503:
WARNING: CPU: 21 PID: 1385 at fs/f2fs/inode.c:762 f2fs_evict_inode+0x847/0xaa0
Modules linked in:
CPU: 21 PID: 1385 Comm: umount Not tainted 5.19.0-rc5+ #73
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-4.fc34 04/01/2014

Call Trace:
evict+0x129/0x2d0
dispose_list+0x4f/0xb0
evict_inodes+0x204/0x230
generic_shutdown_super+0x5b/0x1e0
kill_block_super+0x29/0x80
kill_f2fs_super+0xe6/0x140
deactivate_locked_super+0x44/0xc0
deactivate_super+0x79/0x90
cleanup_mnt+0x114/0x1a0
__cleanup_mnt+0x16/0x20
task_work_run+0x98/0x100
exit_to_user_mode_prepare+0x3d0/0x3e0
syscall_exit_to_user_mode+0x12/0x30
do_syscall_64+0x42/0x80
entry_SYSCALL_64_after_hwframe+0x46/0xb0

Function flow analysis when BUG occurs:
f2fs_fallocate                    mmap
                                  do_page_fault
                                    pte_spinlock  // ---lock_pte
                                    do_wp_page
                                      wp_page_shared
                                        pte_unmap_unlock   // unlock_pte
                                          do_page_mkwrite
                                          f2fs_vm_page_mkwrite
                                            down_read(invalidate_lock)
                                            lock_page
                                            if (PageMappedToDisk(page))
                                              goto out;
                                            // set_page_dirty  --NOT RUN
                                            out: up_read(invalidate_lock);
                                        finish_mkwrite_fault // unlock_pte
f2fs_collapse_range
  down_write(i_mmap_sem)
  truncate_pagecache
    unmap_mapping_pages
      i_mmap_lock_write // down_write(i_mmap_rwsem)
        ......
        zap_pte_range
          pte_offset_map_lock // ---lock_pte
           set_page_dirty
            f2fs_dirty_data_folio
              if (!folio_test_dirty(folio)) {
                                        fault_dirty_shared_page
                                          set_page_dirty
                                            f2fs_dirty_data_folio
                                              if (!folio_test_dirty(folio)) {
                                                filemap_dirty_folio
                                                f2fs_update_dirty_folio // ++
                                              }
                                            unlock_page
                filemap_dirty_folio
                f2fs_update_dirty_folio // page count++
              }
          pte_unmap_unlock  // --unlock_pte
      i_mmap_unlock_write  // up_write(i_mmap_rwsem)
  truncate_inode_pages
  up_write(i_mmap_sem)

When race happens between mmap-do_page_fault-wp_page_shared and
fallocate-truncate_pagecache-zap_pte_range, the zap_pte_range calls
function set_page_dirty without page lock. Besides, though
truncate_pagecache has immap and pte lock, wp_page_shared calls
fault_dirty_shared_page without any. In this case, two threads race
in f2fs_dirty_data_folio function. Page is set to dirty only ONCE,
but the count is added TWICE by calling filemap_dirty_folio.
Thus the count of dirty page cannot accord with the real dirty pages.

Following is the solution to in case of race happens without any lock.
Since folio_test_set_dirty in filemap_dirty_folio is atomic, judge return
value will not be at risk of race.

Signed-off-by: Shuqi Zhang <zhangshuqi3@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>