NFSD: Use set_bit(RQ_DROPME)

The premise that "Once an svc thread is scheduled and executing an
RPC, no other processes will touch svc_rqst::rq_flags" is false.
svc_xprt_enqueue() examines the RQ_BUSY flag in scheduled nfsd
threads when determining which thread to wake up next.

Fixes: 93155647 ("NFSD: Use only RQ_DROPME to signal the need to drop a reply")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>