ofed: fix the WITH_OFED_EXTRA build

This option was not tested when WARNS was globally lifted in the src tree up
to 6. Drop WARNS back down to unbreak the build; note that this is still
enabling more warnings than it had before the WARNS change, so the gcc build
may need to be independently evaluated at this level.

PR: 252865
Reported-by: Build Option Servey via Michael Dexter
MFC-after: 3 days