LU-16498 obdclass: change uc_lock to rwlock

LU-16498 obdclass: change uc_lock to rwlock

Change the upcall cache uc_lock to a read-write lock so that threads
can get the read lock to do concurrent lookups in the upcall cache,
and only grab the write lock in the rare case when a new entry is
added or old entries are expired. That reduces serialization between
server threads during normal operation, and avoids all of the threads
spinning for some time if the requested key (UID or gss context) is
not in the cache at all, before they sleep.

Test-Parameters: kerberos=true testlist=sanity-krb5
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I812400104fd2115d19386fb4a03bb3ce99c49383