namei: make may_follow_link() safe in RCU mode

We *can't* call that audit garbage in RCU mode - it's doing a weird
mix of allocations (GFP_NOFS, immediately followed by GFP_KERNEL)
and I'm not touching that... thing again.

So if this security sclero^Whardening feature gets triggered when
we are in RCU mode, tough - we'll fail with -ECHILD and have
everything restarted in non-RCU mode.  Only to hit the same test
and fail, this time with EACCES and with (oh, rapture) an audit spew
produced.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -794,6 +794,9 @@ static inline int may_follow_link(struct nameidata *nd)
 	if (uid_eq(parent->i_uid, inode->i_uid))
 		return 0;
 
+	if (nd->flags & LOOKUP_RCU)
+		return -ECHILD;
+
 	audit_log_link_denied("follow_link", &nd->stack[0].link);
 	return -EACCES;
 }