RDMA/ipoib: drop skb on path record lookup failure

In unicast_arp_send function there is an inconsistency in error handling
of path_rec_start call. If path_rec_start is called because of an absent
ah field, skb will be dropped. But if it is called on a creation of a
new path, or if the path is invalid, skb will be added to the tail of
path queue.  In case of a new path it will be dropped on path_free, but
in case of invalid path it can stay in the queue forever.

This patch unifies the behavior, dropping skb in all cases
of path_rec_start failure.

Signed-off-by: Evgenii Smirnov <evgenii.smirnov@profitbricks.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>