btrfs: convert copy_inline_to_page() to use folio

btrfs: convert copy_inline_to_page() to use folio

The old page API is being gradually replaced and converted to use folio
to improve code readability and avoid repeated conversion between page
and folio. Moreover find_or_create_page() is compatible API, and it can
replaced with __filemap_get_folio(). Some interfaces have been converted
to use folio before, so the conversion operation from page can be
eliminated here.

Signed-off-by: Li Zetao <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>