nat: fix regarding vm arg for vlib_time_now call

Change in snat_ipfix_header_create() to use thread-specific
vlib_main_t *vm pointer to avoid problems with different threads
accessing the same vlib_main_t data structure. This avoids
assertion failure when vlib_time_now() is called with a vm
corresponding to a different thread.

Type: fix

Signed-off-by: Elias Rudberg <elias.rudberg@bahnhof.net>
Change-Id: I2096c1debb5688d3b97e5ed9a0ea78d94053d8b7