vhost: try to shrink pfdset when fdset_add fails

vhost: try to shrink pfdset when fdset_add fails

fdset_add increments pfdset->num, but fdset_del doesn't decrement
pfdset->num, so if we call fdset_add then fdset_del in a loop without
calling fdset_shrink, we can easily exceed MAX_FDS with only a few
number of fds used.

So my solution is simply to call fdset_shrink in fdset_add when it
exceeds MAX_FDS.

Because fdset_shrink and fdset_add locks pfdset->fd_mutex we can't
call fdset_shrink inside fdset_add because that would cause a dead
lock, so this patch split fdset_shrink in two, fdset_shrink and
fdset_shrink_nolock.

Fixes: 59317cef249c ("vhost: allow many vhost-user ports")
Cc: stable@dpdk.org
Signed-off-by: Matthias Gatto <matthias.gatto@outscale.com>