nat: use correct per-thread vlib_main
nat: use correct per-thread vlib_main

Using vlib_main of another thread is prohibited.

Type: fix

Change-Id: I9a85a5fee5c6665d423b4306faf3b3f2c5e22f99
Signed-off-by: Benoît Ganne <38aad8ffcfa407b890d258e568f161530f66882d@cisco.com>

FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp