lib/dpif-netdev: Make emc_mutex recursive.

lib/dpif-netdev: Make emc_mutex recursive.

dpif_netdev_execute may be called while doing upcall processing.
Since the context of the input port is not tracked upto this point, we
use the shared dp->emc_cache for packet execution, where the emc_cache
is needed for recirculation.

While recursive mutexes can make thread safety analysis hard, for now
we change emc_mutex to be recursive.  Forthcoming new unit tests will
fail with the current non-recursive mutex.  Later improvements may
remove the need for this recursion.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Daniele Di Proietto <ddiproietto@vmware.com>