qa/workunits/rbd-nbd: work around "rbd feature disable" hang

qa/workunits/rbd-nbd: work around "rbd feature disable" hang

"rbd feature disable" appears to reliably hang if the corresponding
remote request is proxied to rbd-nbd (because rbd-nbd happens to own
the exclusive lock after a series of blkdiscard calls) [1].  Work
around it here by enabling journaling before the image is mapped
and disabling it after the image is unmapped.

Also, don't assert on the output of "rbd journal inspect --verbose"
having a certain number of entries.  This is racy: if the script gets
delayed after the last blkdiscard call for some reason, there may be
fewer entries present in the journal or none at all.

[1] https://tracker.ceph.com/issues/58740

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit fcfef0a19e1d85a8b9414909268f1370ec82808d)