nfsd: Make __get_nfsdfs_client() static

Fix sparse warning:

fs/nfsd/nfsctl.c:1221:22: warning:
 symbol '__get_nfsdfs_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>