btrfs: add a size parameter to btrfs_alloc_subpage()

Since we can no longer assume page sized folio for data filemap folios,
allow btrfs_alloc_subpage() to accept a new parameter, @fsize,
indicating the folio size.

This doesn't follow the regular behavior of passing a folio directly,
because this function is shared by both data and metadata folios, and
for metadata folios we have extra allocation policy to ensure no large
folios whose sizes are larger than nodesize (unless it's page sized).

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>