nfscl: Handle CB_SEQUENCE not first op correctly

The check for "not first operation" in CB_SEQUENCE
was done after the slot, etc. was updated. This patch
moves the check to the beginning of CB_SEQUENCE
processing.

While here, also fix the check for "no CB_SEQUENCE operation first"
by moving the check to the beginning of callback operation parsing,
since the check was in a couple of the other operations, but
not all of them.

Reported by: rtm@lcs.mit.edu
Tested by: rtm@lcs.mit.edu
PR: 260412
MFC after: 2 weeks