dpif-netdev: Introduce port_try_ref() to prevent a race.

When pmd thread interates through all ports for queue loading,
the main thread may unreference and 'rcu-free' a port before
pmd thread take new reference of it.  This could cause pmd
thread fail the reference and access freed memory later.

This commit fixes this race by introducing port_try_ref()
which uses ovs_refcount_try_ref_rcu().  And the pmd thread
will only load the port's queue, if port_try_ref() returns
true.

Found by inspection.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>