dlm: use rcu to avoid an extra rsb struct lookup

Use rcu to free rsb structs, and hold the rcu read lock
while looking up rsb structs.  This allows us to avoid an
extra hash table lookup for an rsb.  A new rsb flag HASHED
is added which is set while the rsb is in the hash table.
This flag is checked in place of repeating the hash table
lookup.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>