vlib: fix handoff queue sequencing bug

vlib: fix handoff queue sequencing bug

Set vm->check_frame_queues after actually enqueuing a frame. Under
obscure circumstances, the code managed to set check_frame_queues so
far in advance that 100 dispatch cycles could elapse before the frame
enqueue succeeded. That resulted in permanent lack of queue service.

Type: fix
Ticket: VPP-1734
Fixes: 18191

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: If2d398202b4ba2b96581d25e8142daef3f74c9e5
(cherry picked from commit 320998a8126766c0db92d9c7652f765f9565a7b5)