netdev-dpdk: Fix locking during get_stats.

netdev-dpdk: Fix locking during get_stats.

Clang complains:
lib/netdev-dpdk.c:1860:1: error: mutex 'dev->mutex' is not locked on every path
      through here [-Werror,-Wthread-safety-analysis]
}
^
lib/netdev-dpdk.c:1815:5: note: mutex acquired here
    ovs_mutex_lock(&dev->mutex);
    ^
./include/openvswitch/thread.h:60:9: note: expanded from macro 'ovs_mutex_lock'
        ovs_mutex_lock_at(mutex, OVS_SOURCE_LOCATOR)
        ^

Fixes: d6e3feb57c44 ("Add support for extended netdev statistics based on RFC 2819.")
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>