rbd-nbd: fix potential race when waiting unqiesce

rbd-nbd: fix potential race when waiting unqiesce

There was a window between issuing quiesce_complete and checking
the quiesce variable unset by the unqiesce callback, when the lock
was not holding. If during that window the unquiesce following the
next quiesce callbacks were called we would miss unquiesce event.

Signed-off-by: Mykola Golub <mgolub@suse.com>