infiniband: call ipv6 route lookup via the stub interface

The infiniband address handle can be triggered to resolve an ipv6
address in response to MAD packets, regardless of the ipv6
module being disabled via the kernel command line argument.

That will cause a call into the ipv6 routing code, which is not
initialized, and a conseguent oops.

This commit addresses the above issue replacing the direct lookup
call with an indirect one via the ipv6 stub, which is properly
initialized according to the ipv6 status (e.g. if ipv6 is
disabled, the routing lookup fails gracefully)

Cc: stable@vger.kernel.org # 3.12+
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>