report the potential selinux problem if mmap_seized() fails

selinux can deny mmap(PROT_WRITE | PROT_EXEC) and in this case it is
not clear why CRIU fails, "Can't allocate memory for parasite blob"
doesn't tell too much. Add a pr_warn() hint for the user.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Cyrill Gorcunov<gorcunov@openvz.org>
Acked-by: Ruslan Kuprieiev <rkuprieiev@cloudlinux.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

arch/x86/crtools.c

@@ -445,8 +445,15 @@ void *mmap_seized(struct parasite_ctl *ctl,
 
 	err = syscall_seized(ctl, __NR_mmap, &map,
 			(unsigned long)addr, length, prot, flags, fd, offset);
-	if (err < 0 || map > TASK_SIZE)
-		map = 0;
+	if (err < 0)
+		return NULL;
+
+	if (IS_ERR_VALUE(map)) {
+		if (map == -EACCES && (prot & PROT_WRITE) && (prot & PROT_EXEC))
+			pr_warn("mmap(PROT_WRITE | PROT_EXEC) failed for %d, "
+				"check selinux execmem policy\n", ctl->pid.real);
+		return NULL;
+	}
 
 	return (void *)map;
 }