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: eccbbc7c ("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>