Improve error msg on idmap mounts

Some filesystems don't support idmap mounts and just showing to the user
invalid argument is not super clear.

When errno is EINVAL, let's add a text hinting that the cause could be
that idmap mounts are not supported on that filesystem used.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

src/libcrun/linux.c

@@ -4018,6 +4018,7 @@ maybe_get_idmapped_mount (runtime_spec_schema_config_schema *def, runtime_spec_s
   char proc_path[64];
   bool has_mappings;
   int ret;
+  char *extraMsg = "";
 
   *out_fd = -1;
 
@@ -4081,8 +4082,12 @@ maybe_get_idmapped_mount (runtime_spec_schema_config_schema *def, runtime_spec_s
   attr.userns_fd = fd;
 
   ret = syscall_mount_setattr (newfs_fd, "", AT_EMPTY_PATH | (recursive ? AT_RECURSIVE : 0), &attr);
-  if (UNLIKELY (ret < 0))
-    return crun_make_error (err, errno, "mount_setattr `%s`", mnt->destination);
+  if (UNLIKELY (ret < 0)) {
+    if (errno == EINVAL) {
+      extraMsg = "(maybe the filesystem used doesn't support idmap mounts on this kernel?)";
+    }
+    return crun_make_error (err, errno, "mount_setattr `%s` %s", mnt->destination, extraMsg);
+  }
 
   *out_fd = get_and_reset (&newfs_fd);
   return 0;