Update after lookup in map.increment for HASH types

Performing the update before lookup incurs an extra spinlock in the
kernel, which hurts performance. Reorder the code and increment to 1.
The memset is there I believe due to some type mismatch warnings, so I'm
leaving as is rather than doing a direct assign to 1. The resulting code
is optimized away anyway.

Fixes: #1314
Signed-off-by: Brenden Blanco <bblanco@gmail.com>

src/cc/frontends/clang/b_frontend_action.cc

@@ -332,12 +332,14 @@ bool BTypeVisitor::VisitCallExpr(CallExpr *Call) {
           string lookup = "bpf_map_lookup_elem_(bpf_pseudo_fd(1, " + fd + ")";
           string update = "bpf_map_update_elem_(bpf_pseudo_fd(1, " + fd + ")";
           txt  = "({ typeof(" + name + ".key) _key = " + arg0 + "; ";
+          txt += "typeof(" + name + ".leaf) *_leaf = " + lookup + ", &_key); ";
+          txt += "if (_leaf) (*_leaf)++; ";
           if (desc->second.type == BPF_MAP_TYPE_HASH) {
-            txt += "typeof(" + name + ".leaf) _zleaf; memset(&_zleaf, 0, sizeof(_zleaf)); ";
-            txt += update + ", &_key, &_zleaf, BPF_NOEXIST); ";
+            txt += "else { typeof(" + name + ".leaf) _zleaf; memset(&_zleaf, 0, sizeof(_zleaf)); ";
+            txt += "_zleaf++; ";
+            txt += update + ", &_key, &_zleaf, BPF_NOEXIST); } ";
           }
-          txt += "typeof(" + name + ".leaf) *_leaf = " + lookup + ", &_key); ";
-          txt += "if (_leaf) (*_leaf)++; })";
+          txt += "})";
         } else if (memb_name == "perf_submit") {
           string name = Ref->getDecl()->getName();
           string arg0 = rewriter_.getRewrittenText(expansionRange(Call->getArg(0)->getSourceRange()));