Revert "SUNRPC: Use RMW bitops in single-threaded hot paths"

Revert "SUNRPC: Use RMW bitops in single-threaded hot paths"

[ Upstream commit 7827c81f0248e3c2f40d438b020f3d222f002171 ]

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.

Found via KCSAN.

Fixes: 28df0988815f ("SUNRPC: Use RMW bitops in single-threaded hot paths")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>