librbd: fixed deadlock while attempting to flush AIO requests

librbd: fixed deadlock while attempting to flush AIO requests

In-flight AIO requests might force a flush if a snapshot was created
out-of-band.  The flush completion was previously invoked asynchronously,
potentially via the same thread worker handling the AIO request. This
resulted in the flush operation deadlocking since it can't complete.

Fixes: #13726
Backport: infernalis, hammer
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit bfeb90e5fe24347648c72345881fd3d932243c98)