nfscl: Sanity check the callback tag length

The sanity check for tag length in a callback request
was broken in two ways:

It checked for a negative value, but not a large positive
value.

It did not set taglen to -1, to indicate to the code that
it should not be used.

This patch fixes both of these issues.

Reported by: rtm@lcs.mit.edu
Tested by: rtm@lcs.mit.edu
PR: 260266
MFC after: 2 weeks