svcrdma: Poll CQs in "workqueue" mode

svcrdma calls svc_xprt_put() in its completion handlers, which
currently run in IRQ context.

However, svc_xprt_put() is meant to be invoked in process context,
not in IRQ context. After the last transport reference is gone, it
directly calls a transport release function that expects to run in
process context.

Change the CQ polling modes to IB_POLL_WORKQUEUE so that svcrdma
invokes svc_xprt_put() only in process context. As an added benefit,
bottom half-disabled spin locking can be eliminated from I/O paths.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>