btrfs: remove btrfs_fs_info::sectors_per_page

btrfs: remove btrfs_fs_info::sectors_per_page

For the future large folio support, our filemap can have folios with
different sizes, thus we can no longer rely on a fixed blocks_per_page
value.

To prepare for that future, here we do:

- Remove btrfs_fs_info::sectors_per_page

- Introduce a helper, btrfs_blocks_per_folio()
  Which uses the folio size to calculate the number of blocks for each
  folio.

- Migrate the existing btrfs_fs_info::sectors_per_page to use that
  helper
  There are some exceptions:

  * Metadata nodesize < page size support
    In the future, even if we support large folios, we will only
    allocate a folio that matches our nodesize.
    Thus we won't have a folio covering multiple metadata unless
    nodesize < page size.

  * Existing subpage bitmap dump
    We use a single unsigned long to store the bitmap.
    That means until we change the bitmap dumping code, our upper limit
    for folio size will only be 256K (4K block size, 64 bit unsigned
    long).

  * btrfs_is_subpage() check
    This will be migrated into a future patch.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>