nfsd: Add missing gen_confirm in nfsd4_setclientid()

Commit 294ac32e "nfsd: protect clid and verifier generation with
client_lock" moved gen_confirm() to gen_clid().

After that commit, setclientid will return a bad reply with all-zero
verifier after copy_clid().

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>