nfsd: Fix old-style function definition

Fix warning:

fs/nfsd/nfssvc.c:604:6: warning: old-style function definition [-Wold-style-definition]
 bool i_am_nfsd()
      ^~~~~~~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ma Feng <mafeng.ma@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>