dpif-netdev: Remove PMD latency on seq_mutex

dpif-netdev: Remove PMD latency on seq_mutex

The PMD thread needs to keep processing RX queues in order
to achieve maximum throughput. It also needs to sweep emc
cache and quiesce which use seq_mutex. That mutex can
eventually block the PMD thread causing latency spikes and
affecting the throughput.

Since there is no requirement for running those tasks at a
specific time, this patch extend seq API to allow tentative
locking instead.

Reported-by: Karl Rister <krister@redhat.com>
Co-authored-by: Karl Rister <krister@redhat.com>
Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>