infiniband: avoid dereferencing uninitialized dst on error path

With commit eea40b8f ("infiniband: call ipv6 route lookup
via the stub interface"), if the route lookup fails due to
ipv6 being disabled, the dst variable is left untouched, and
the following dst_release() may access uninitialized memory.

Since ipv6_dst_lookup() always sets dst to NULL in case of
lookup failure with ipv6 enabled, fix the above just
returning the error code if the lookup fails.

Fixes: eea40b8f ("infiniband: call ipv6 route lookup via the stub interface")
Reported-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Doug Ledford <dledford@redhat.com>