rbd: use a single workqueue for all devices
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 <152d4c5bebf13a5b809d42b4d789cf9d887e6698@redhat.com>
Reviewed-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@redhat.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs