net: mctp: put sock on tag allocation failure

We may hold an extra reference on a socket if a tag allocation fails: we
optimistically allocate the sk_key, and take a ref there, but do not
drop if we end up not using the allocated key.

Ensure we're dropping the sock on this failure by doing a proper unref
rather than directly kfree()ing.

Fixes: de8a6b15 ("net: mctp: add an explicit reference from a mctp_sk_key to sock")
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/ce9b61e44d1cdae7797be0c5e3141baf582d23a0.1707983487.git.jk@codeconstruct.com.auSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mctp/route.c

@@ -663,7 +663,7 @@ struct mctp_sk_key *mctp_alloc_local_tag(struct mctp_sock *msk,
 	spin_unlock_irqrestore(&mns->keys_lock, flags);
 
 	if (!tagbits) {
-		kfree(key);
+		mctp_key_unref(key);
 		return ERR_PTR(-EBUSY);
 	}