nfsd: use true,false for bool variable in nfssvc.c

Fixes coccicheck warning:

fs/nfsd/nfssvc.c:394:2-14: WARNING: Assignment of 0/1 to bool variable
fs/nfsd/nfssvc.c:407:2-14: WARNING: Assignment of 0/1 to bool variable
fs/nfsd/nfssvc.c:422:2-14: WARNING: Assignment of 0/1 to bool variable

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>