yama: Better permission check for ptraceme

Change the permission check for yama_ptrace_ptracee to the standard
ptrace permission check, testing if the traceer has CAP_SYS_PTRACE
in the tracees user namespace.

Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: "Eric W. Biederman" <[e-mail]>

security/yama/yama_lsm.c

···
 	/* Only disallow PTRACE_TRACEME on more aggressive settings. */
 	switch (ptrace_scope) {
 	case YAMA_SCOPE_CAPABILITY:
-		rcu_read_lock();
-		if (!ns_capable(__task_cred(parent)->user_ns, CAP_SYS_PTRACE))
+		if (!has_ns_capability(parent, current_user_ns(), CAP_SYS_PTRACE))
 			rc = -EPERM;
-		rcu_read_unlock();
 		break;
 	case YAMA_SCOPE_NO_ATTACH:
 		rc = -EPERM;