flowprobe: fix calling vlib_time_now() from worker threads

flowprobe: fix calling vlib_time_now() from worker threads

Currently, when flowprobe_export_send() calls vlib_time_now(), a pointer
to the main thread's vlib_main_t is always passed (the one cached in
flow_report_main). However, that code can also be executed from a worker
thread. And passing a pointer to the main thread's vlib_main_t to
vlib_time_now() from a worker thread may cause time synchronization
issues. Also, running a debug binary will cause an assertion failure in
vlib_time_now() in this case.

With this fix, flowprobe_export_send() passes the pointer to the current
thread's vlib_main_t to vlib_time_how().

This doesn't allow to remove @tag_fixme_vpp_workers from the unit tests
yet as they will be failing for other multi-worker related problems.

Type: fix
Change-Id: Ia35e3a4176777b88cf8ca8af8af7c42c495cbc6a
Signed-off-by: Alexander Chernavin <achernavin@netgate.com>