nfsd: fix __fh_verify for localio

__fh_verify() added a call to svc_xprt_set_valid() to help do connection
management but during LOCALIO path rqstp argument is NULL, leading to
NULL pointer dereferencing and a crash.

Fixes: eccbbc7 ("nfsd: don't use sv_nrthreads in connection limiting calculations.")
Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: Olga Kornievskaia

fs/nfsd/nfsfh.c

@@ -381,8 +381,9 @@ __fh_verify(struct svc_rqst *rqstp,
 	error = check_nfsd_access(exp, rqstp, may_bypass_gss);
 	if (error)
 		goto out;
-
-	svc_xprt_set_valid(rqstp->rq_xprt);
+	/* During LOCALIO call to fh_verify will be called with a NULL rqstp */
+	if (rqstp)
+		svc_xprt_set_valid(rqstp->rq_xprt);
 
 	/* Finally, check access permissions. */
 	error = nfsd_permission(cred, exp, dentry, access);