nfsd: declare several functions of nfs4callback as static

setup_callback_client(), nfsd4_release_cb() and nfsd4_process_cb_update()
do not have users outside the translation unit. Let's declare it as
static.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>