irdma(4): use related vnet for searching netdev

It was found through testing that when ULP uses individual vnet, the
search for the correct vlan_id may failing because of no proper
interface with given address.

The solution is to use vnet associated to the connection whenever
possible.

Signed-off-by: Bartosz Sobczak <bartosz.sobczak@intel.com>
Signed-off-by: Eric Joyner <erj@FreeBSD.org>

Reviewed by: erj@
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D41592

(cherry picked from commit bc3a013ccd23c50bb48f17211ffc36fe85d11e18)