btrfs: make btrfs_dec_test_first_ordered_pending take btrfs_inode

It doesn't really need vfs_inode but btrfs_inode.

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