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>