btrfs: Remove fs_info from insert_inline_extent_backref

It can be referenced from the passed transaction handle, since it's
always valid.

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