librados: take lock when signaling notify cond

librados: take lock when signaling notify cond

When we are signaling the cond to indicate that a notify is complete,
take the appropriate lock.  This removes the possibility of a race
that loses our signal.  (That would be very difficult given that there
are network round trips involved, but this makes the lock/cond usage
"correct.")

Signed-off-by: Sage Weil <sage@inktank.com>