RDMA/ucma: Allow resolving address w/o specifying source address

The RDMA CM will select a source device and address by consulting
the routing table if no source address is passed into
rdma_resolve_address().  Userspace will ask for this by passing an
all-zero source address in the RESOLVE_IP command.  Unfortunately
the new check for non-zero address size rejects this with EINVAL,
which breaks valid userspace applications.

Fix this by explicitly allowing a zero address family for the source.

Fixes: 2975d5de ("RDMA/ucma: Check AF family prior resolving address")
Cc: <stable@vger.kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>