vhost: fix race condition when adding fd in the fdset

vhost: fix race condition when adding fd in the fdset

fdset_add can call fdset_shrink_nolock which call fdset_move
concurrently to poll that is call in fdset_event_dispatch.

This patch add a mutex to protect poll from been call at the same time
fdset_add call fdset_shrink_nolock.

Fixes: 1b815b89599c ("vhost: try to shrink pfdset when fdset_add fails")
Cc: stable@dpdk.org
Signed-off-by: Matthias Gatto <matthias.gatto@outscale.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>