nfsd: Make two functions static

Fix sparse warnings:

fs/nfsd/nfs4state.c:1908:6: warning: symbol 'drop_client' was not declared. Should it be static?
fs/nfsd/nfs4state.c:2518:6: warning: symbol 'force_expire_client' 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>