nfsd: Make nfsd_reset_boot_verifier_locked static

Fix sparse warning:

fs/nfsd/nfssvc.c:364:6: warning:
 symbol 'nfsd_reset_boot_verifier_locked' was not declared. Should it be static?

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