libceph: ceph-msgr workqueue needs a resque worker

commit f9865f06f7f18c6661c88d0511f05c48612319cc upstream.

Commit f363e45fd118 ("net/ceph: make ceph_msgr_wq non-reentrant")
effectively removed WQ_MEM_RECLAIM flag from ceph_msgr_wq.  This is
wrong - libceph is very much a memory reclaim path, so restore it.

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
Tested-by: Micha Krause <micha@krausam.de>
Reviewed-by: Sage Weil <sage@redhat.com>
[bwh: Backported to 3.2:
 - Keep passing the WQ_NON_REENTRANT flag too
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>