svcrpc: don't bother checking bad svc_addr_len result

None of the callers should see an unsupported address family (only one
of them even bothers to check for that case), so just check for the
buggy case in svc_addr_len and don't bother elsewhere.

Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>