datapath: Eliminate vport_mutex by protecting vport table with RCU.

datapath: Eliminate vport_mutex by protecting vport table with RCU.

The vport_mutex really only protects the vport dev_table, which isn't very
much.  By getting rid of it we take one step toward simplifying the vswitch
locking, which will necessarily have to be based mainly around the Generic
Netlink genl_mutex once we switch to Generic Netlink.

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