common/attr: fix the _require_acl test

_require_acl tests whether you're able to fetch the ACL from a file
using chacl. Unfortunately, filesystems like NFSv4 return -ENODATA when
someone calls getxattr for system.posix_acl. chacl treats that as if
there is no ACL attached to the file and so doesn't return an error
even though the filesystem doesn't support ACLs.

Fix the test to have chacl set an ACL on the file instead, which should
reliably fail on filesystems that don't support them.

Signed-off-by: Jeff Layton <jlayton@kernel.org>