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

(cherry picked from commit 9ca71db4959f674d75f7b4fd0f72bdc1e2e99abb)