nfsd: calculate the missing length of bitmap in EXCHANGE_ID

commit 58cd57bf
"nfsd: Fix SP4_MACH_CRED negotiation in EXCHANGE_ID"
miss calculating the length of bitmap for spo_must_enforce and spo_must_allow.

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