librbd: defer event socket completion until after callback issued

librbd: defer event socket completion until after callback issued

A change post-Nautilus fixed an issue where multiple threads could
concurrently invoke callbacks to librbd clients. However, it also
introduced the potential that a callback hasn't yet fired by the
time the event socket is completed. This resulted in a crash of
fio under high-throughput testing since it expected both a callback
and the event socket completion, in that order.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>