btrfs: convert defrag_prepare_one_page() to use a folio
Use a folio throughout defrag_prepare_one_page() to remove dozens of
hidden calls to compound_head(). There is no support here for large
folios; indeed, turn the existing check for PageCompound into a check
for large folios.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>