vfs: don't copy things to user space holding the rcu readlock

Oops.  That wasn't very smart.  We don't actually need the RCU lock any
more by the time we copy the cwd string to user space, but I had
stupidly surrounded the whole thing with it.

Introduced by commit 8b19e34188 ("vfs: make getcwd() get the root and
pwd path under rcu")

Is-a-big-hairy-idiot: Linus Torvalds <torvalds@linux-foundation.org>

fs/dcache.c

@@ -3067,6 +3067,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
 		prepend(&cwd, &buflen, "\0", 1);
 		error = prepend_path(&pwd, &root, &cwd, &buflen);
 		br_read_unlock(&vfsmount_lock);
+		rcu_read_unlock();
 
 		if (error < 0)
 			goto out;
@@ -3087,10 +3088,10 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
 		}
 	} else {
 		br_read_unlock(&vfsmount_lock);
+		rcu_read_unlock();
 	}
 
 out:
-	rcu_read_unlock();
 	free_page((unsigned long) page);
 	return error;
 }