snabbmark esp: minor changes

 * add a note that explains performance differences between esp transport and
   tunnel modes

 * explicitly return a single value from `decap' in tunnel mode (because of
   that thing where LuaJIT can trip over this)

...these changes are mostly notes to prevent future selfs from wondering about
this again.

src/program/snabbmark/README

@@ -45,6 +45,10 @@ Usage:
     Optionally, a LuaJIT profiler option string can be supplied as <profile>,
     which will cause the benchmark run to be profiled accordingly.
 
+    Note: transport mode may generally score better because it has to
+    encapsulate 40 bytes less per packet because of the enclosed IPv6 header
+    in tunnel mode, and this micro-benchmark measures end-to-end throughput.
+
   snabbmark hash [<key-size>]
     Benchmark hash functions used for internal data structures.
 

src/program/snabbmark/snabbmark.lua

@@ -372,7 +372,7 @@ function esp (npackets, packet_size, mode, direction, profile)
    local encap, decap
    if mode == "tunnel" then
       encap = function (p) return enc:encapsulate_tunnel(p, 41) end
-      decap = function (p) return dec:decapsulate_tunnel(p) end
+      decap = function (p) return (dec:decapsulate_tunnel(p))   end
    else
       encap = function (p) return enc:encapsulate_transport6(p) end
       decap = function (p) return dec:decapsulate_transport6(p) end