btrfs: migrate insert_inline_extent() to folio interfaces

Since insert_inline_extent() now only accepts a single page, it's much
easier to convert it to use folio interfaces.

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