revalidator: Protect ukeys with a mutex.

revalidator: Protect ukeys with a mutex.

Currently, udpif_keys are protected during revalidator_sweep__() as only
one thread accesses the ukey at a time. This is ensured using barriers:
all revalidators will be in the GC phase, so they will only access their
own ukey collection.

A future patch will change the access patterns to allow these ukey
collections to be read or modified while a revalidator is garbage
collecting it. To protect the ukeys, this patch adds locking on the ukey
collection.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>