Provide a dead version of if_get_counter.

[jhb]

Test Plan

Configure a T4/T5 interface and run 'netstat -hwd 1' in one window while
doing 'kldunload if_cxgbe' in another. This triggers a panic in
if_data_copy(). This patch doesn't remove the panic entirely, but it
does seem to help it occur less often.

[glebius]

Ok from me. Anyway in my new ifnet branch if_dead goes away.

[jhb]

Forgot to note in the commit log. This was committed in r275730.