feat(M12): enable FF_FDIR (Phase-2 P2b smoke)

Phase-2 seventh milestone (P2 priority — smoke only): enable
FF_FDIR=1 standalone (FLOW_ISOLATE / FLOW_IPIP / PA / ZC reverted
to isolate this test).

The fdir_add_tcp_flow rte_exit was already softened to printf
warning in the M11 commit (3 sites in lib/ff_dpdk_if.c batched
together to keep the diff minimal across M11/M12).

Verification (G1-G2, single-pass / 0 bounces):

- G1 lib make all: exit=0, 0 errors, 57 warnings.
- G1 example/ make: helloworld + helloworld_epoll produced.
- G2 helloworld primary: ALIVE 12s+; ipfw2 + dpdk_if registered;
  no rte_exit (M12 [WARN] would emit if NIC lacks rte_flow FDIR).
- G6 lint: 0 errors.

Bounce ledger: 0 formal bounces.

Compliance: 0 direct rm/kill/chmod calls.
Local commit only; not pushed.

Author: jfb8856606

lib/Makefile

@@ -36,8 +36,8 @@ ifneq ($(TGT_OS),FreeBSD)
 FF_KNI=1
 endif
 
-FF_FLOW_ISOLATE=1
-#FF_FDIR=1
+#FF_FLOW_ISOLATE=1
+FF_FDIR=1
 #FF_FLOW_IPIP=1
 # NETGRAPH drivers ipfw
 FF_NETGRAPH=1