mknodat: refuse to create symlinks

The current mknodat code creates symlinks when requested. Not only does
this break POSIX, it also causes a kernel crash (NULL-deref on
tmpfs_readlink).

Refuse to create symlinks with mknodat.

Found by syzkaller fuzzing.

kernel-src/sys/syscalls/mknodat.c

@@ -34,6 +34,11 @@ syscallret_t syscall_mknodat(context_t *, int dirfd, char *upath, mode_t mode, d
 		goto cleanup;
 
 	int type = vfs_getsystemtype(GETTYPE(mode));
+	if (type == V_TYPE_LINK) {
+		ret.errno = EINVAL;
+		goto cleanup;
+	}
+
 	mode = GETMODE(mode);
 
 	vattr_t attr = {