rxrpc: checking for IS_ERR() instead of NULL

The rxrpc_lookup_peer() function returns NULL on error, it never returns
error pointers.

Fixes: 8496af50 ('rxrpc: Use RCU to access a peer's service connection tree')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>