nfsd: fix auth_domain reference leak on nlm operations

This was noticed by users who performed more than 2^32 lock operations
and hence made this counter overflow (eventually leading to
use-after-free's).  Setting rq_client to NULL here means that it won't
later get auth_domain_put() when it should be.

Appears to have been introduced in 2.5.42 by "[PATCH] kNFSd: Move auth
domain lookup into svcauth" which moved most of the rq_client handling
to common svcauth code, but left behind this one line.

Cc: Neil Brown <neilb@suse.de>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/lockd.c

@@ -38,7 +38,6 @@ nlm_fopen(struct svc_rqst *rqstp, struct nfs_fh *f, struct file **filp)
 	exp_readlock();
 	nfserr = nfsd_open(rqstp, &fh, S_IFREG, NFSD_MAY_LOCK, filp);
 	fh_put(&fh);
-	rqstp->rq_client = NULL;
 	exp_readunlock();
  	/* We return nlm error codes as nlm doesn't know
 	 * about nfsd, but nfsd does know about nlm..