xprtrdma: Do not refresh Receive Queue while it is draining

Currently the Receive completion handler refreshes the Receive Queue
whenever a successful Receive completion occurs.

On disconnect, xprtrdma drains the Receive Queue. The first few
Receive completions after a disconnect are typically successful,
until the first flushed Receive.

This means the Receive completion handler continues to post more
Receive WRs after the drain sentinel has been posted. The late-
posted Receives flush after the drain sentinel has completed,
leading to a crash later in rpcrdma_xprt_disconnect().

To prevent this crash, xprtrdma has to ensure that the Receive
handler stops posting Receives before ib_drain_rq() posts its
drain sentinel.

Suggested-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>