MINOR: hlua: take the global Lua lock inside a global function

MINOR: hlua: take the global Lua lock inside a global function

Some users are facing huge CPU usage or even watchdog panics due to
the Lua global lock when many threads compete on it, but they have
no way to see that in the usual dumps. We take the lock at 2 or 3
places only, thus it's trivial to move it to a global function so
that stack dumps will now explicitly show it, increasing the change
that it rings a bell and someone suggests switch to lua-load-per-thread:

  Current executing Lua from a stream analyser -- stack traceback:
      loop.lua:1: in function line 1
  call trace(27):
  |       0x5ff157 [48 83 c4 10 5b 5d 41 5c]: wdt_handler+0xf7/0x104
  | 0x7fe37fe82690 [48 c7 c0 0f 00 00 00 0f]: libpthread:+0x13690
  |       0x614340 [66 48 0f 7e c9 48 01 c2]: main+0x1e8a40
  |       0x607b85 [48 83 c4 08 48 89 df 31]: main+0x1dc285
  |       0x6070bc [48 8b 44 24 20 48 8b 14]: main+0x1db7bc
  |       0x607d37 [41 89 c4 89 44 24 1c 83]: lua_resume+0xc7/0x214
  |       0x464ad6 [83 f8 06 0f 87 f1 01 00]: main+0x391d6
  |       0x4691a7 [83 f8 06 0f 87 03 20 fc]: main+0x3d8a7
  |       0x51dacb [85 c0 74 61 48 8b 5d 20]: sample_process+0x4b/0xf7
  |       0x51e55c [48 85 c0 74 3f 64 48 63]: sample_fetch_as_type+0x3c/0x9b
  |       0x525613 [48 89 c6 48 85 c0 0f 84]: sess_build_logline+0x2443/0x3cae
  |       0x4af0be [4c 63 e8 4c 03 6d 10 4c]: http_apply_redirect_rule+0xbfe/0xdf8
  |       0x4af523 [83 f8 01 19 c0 83 e0 03]: main+0x83c23
  |       0x4b2326 [83 f8 07 0f 87 99 00 00]: http_process_req_common+0xf6/0x15f6
  |       0x4d5b30 [85 c0 0f 85 9f f5 ff ff]: process_stream+0x2010/0x4e18

It also allows "perf top" to directly show the time spent on this lock.

This may be backported to some stable versions as it improves the
overall debuggability.