six locks: Disable percpu read lock mode in userspace

When running in userspace, we currently don't have a real percpu
implementation available - at least in bcachefs-tools, which is where
this code is currently used in userspace.

Signed-off-by: Kent Overstreet <[e-mail]>

fs/bcachefs/six.c

···
 	lockdep_init_map(&lock->dep_map, name, key, 0);
 #endif
 
+	/*
+	 * Don't assume that we have real percpu variables available in
+	 * userspace:
+	 */
+#ifdef __KERNEL__
 	if (flags & SIX_LOCK_INIT_PCPU) {
 		/*
 		 * We don't return an error here on memory allocation failure
···
 		 */
 		lock->readers = alloc_percpu(unsigned);
 	}
+#endif
 }
 EXPORT_SYMBOL_GPL(__six_lock_init);