selftests/mount_setattr: fix idmap_mount_tree_invalid failed to run

selftests/mount_setattr: fix idmap_mount_tree_invalid failed to run

Test case idmap_mount_tree_invalid failed to run on the newer kernel
with the following output:

 #  RUN           mount_setattr_idmapped.idmap_mount_tree_invalid ...
 # mount_setattr_test.c:1428:idmap_mount_tree_invalid:Expected sys_mount_setattr(open_tree_fd, "", AT_EMPTY_PATH, &attr,  sizeof(attr)) (0) ! = 0 (0)
 # idmap_mount_tree_invalid: Test terminated by assertion

This is because tmpfs is mounted at "/mnt/A", and tmpfs already
contains the flag FS_ALLOW_IDMAP after the commit 7a80e5b8c6fa ("shmem:
support idmapped mounts for tmpfs"). So calling sys_mount_setattr here
returns 0 instead of -EINVAL as expected.

Ramfs does not support idmap mounts, so we can use it here to test invalid mounts,
which allows the test case to pass with the following output:

 # Starting 1 tests from 1 test cases.
 #  RUN           mount_setattr_idmapped.idmap_mount_tree_invalid ...
 #            OK  mount_setattr_idmapped.idmap_mount_tree_invalid
 ok 1 mount_setattr_idmapped.idmap_mount_tree_invalid
 # PASSED: 1 / 1 tests passed.

Link: https://lore.kernel.org/all/20241028084132.3212598-1-zhouyuhang1010@163.com/
Signed-off-by: zhouyuhang <zhouyuhang@kylinos.cn>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>