f2fs: fix to wait page writeback before setting gcing flag

f2fs: fix to wait page writeback before setting gcing flag

Soft IRQ				Thread
- f2fs_write_end_io
					- f2fs_defragment_range
					 - set_page_private_gcing
 - type = WB_DATA_TYPE(page, false);
 : assign type w/ F2FS_WB_CP_DATA
 due to page_private_gcing() is true
  - dec_page_count() w/ wrong type
  - end_page_writeback()

Value of F2FS_WB_CP_DATA reference count may become negative under above
race condition, the root cause is we missed to wait page writeback before
setting gcing page private flag, let's fix it.

Fixes: 2d1fe8a86bf5 ("f2fs: fix to tag gcing flag on page during file defragment")
Fixes: 4961acdd65c9 ("f2fs: fix to tag gcing flag on page during block migration")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>