fix the decap macro

Summary:
the encap is always gue so it doesn't make sense to use INLINE_DECAP_IPIP. this flavor is not used in production today.
Changed it to INLINE_DECAP_GUE and added unit tests for decap.

buck run //kernel/bpf-validation/gen_bpf_validation_targets:gen_bpf_validation_targets

Reviewed By: p-shah256

Differential Revision: D92069194

fbshipit-source-id: 98cb2d6fc94ec6b396d87f5f2dda882440e895e4

Author: Fei Chen

katran/lib/bpf/balancer_consts.h

@@ -383,14 +383,6 @@ v2 tracks misses for TCP non syns */
  * LOCAL_DELIVERY_OPTIMIZATION - allow to do optimization on local traffic,
  * where vip and real address are specified the same machine
  */
-#ifdef LPM_SRC_LOOKUP
-#ifndef INLINE_DECAP
-#ifndef INLINE_DECAP_GUE
-#define INLINE_DECAP
-#endif // of INLINE_DECAP_GUE
-#endif // of INLINE_DECAP
-#endif // of LPM_SRC_LOOKUP
-
 #ifdef INLINE_DECAP
 #ifndef INLINE_DECAP_IPIP
 #define INLINE_DECAP_IPIP