ofproto: Replace rwlock in struct rule by a mutex.

ofproto: Replace rwlock in struct rule by a mutex.

A rwlock is suitable when one expects long hold times so there is a need
for some parallelism for readers.  But when the lock is held briefly, it
makes more sense to use a mutex for two reasons.  First, a rwlock is more
complex than a mutex so one would expect it to be more expensive to
acquire.  Second, a rwlock is less fair than a mutex: as long as there are
any readers this blocks out writers.

At least, that's the behavior I intuitively expect, and a few looks around
the web suggest that I'm not the only one.

Previously, struct rule's rwlock was held for long periods, so using a
rwlock made sense.  Now it is held only briefly, so this commit replaces it
by a mutex.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>