devtmpfs: create private mount namespace

Kernel threads start in a completely isolated nullfs mount. Use
UNSHARE_EMPTY_MNTNS to give the devtmpfsd kthread a private empty mount
namespace with its root and pwd already set up, so it can mount its own
devtmpfs instance instead of unsharing a copy of the initial mount
namespace.

Link: https://patch.msgid.link/[email protected]
Signed-off-by: Christian Brauner (Amutable) <[email protected]>

drivers/base/devtmpfs.c

@@ -413,7 +413,7 @@ static noinline int __init devtmpfs_setup(void *p)
 {
 	int err;
 
-	err = ksys_unshare(CLONE_NEWNS);
+	err = ksys_unshare(UNSHARE_EMPTY_MNTNS);
 	if (err)
 		goto out;
 	err = init_mount("devtmpfs", "/", "devtmpfs", DEVTMPFS_MFLAGS, NULL);