datapath: Convert patch vport to use call_rcu() on destruction.

datapath: Convert patch vport to use call_rcu() on destruction.

Since patch ports are virtual devices, we can potentially have many
of them in a datapath.  Currently we have a call to synchronize_rcu()
each time we destroy one, which can be expensive if we are deleting a
datapath with many ports.  This converts it to use call_rcu() instead,
which allows us to wait for only a single RCU grace period independent
of the number of ports.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>