rxrpc: Fix checking of error from ip6_route_output()

ip6_route_output() doesn't return a negative error when it fails, rather
the ->error field of the returned dst_entry struct needs to be checked.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 75b54cb5 ("rxrpc: Add IPv6 support")
Signed-off-by: David Howells <dhowells@redhat.com>