libceph: ceph-msgr workqueue needs a resque worker

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.

Cc: stable@vger.kernel.org # needs backporting for < 3.12
Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
Tested-by: Micha Krause <micha@krausam.de>
rbd: rbd workqueues need a resque worker

Need to use WQ_MEM_RECLAIM for our workqueues to prevent I/O lockups
under memory pressure - we sit on the memory reclaim path.

Cc: stable@vger.kernel.org # 3.17, needs backporting for 3.16
Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
Tested-by: Micha Krause <micha@krausam.de>
rbd: use a single workqueue for all devices

Using one queue per device doesn't make much sense given that our
workfn processes "devices" and not "requests".  Switch to a single
workqueue for all devices.

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>