staging: erofs: Add identifier for function definition arguments

Add identifier for function definition arguments in xattr_iter_handlers,
this change clears the checkpatch.pl issue and make code more explicit.

Signed-off-by: Sidong Yang <realwakka@gmail.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>