sunrpc: make token header values less confusing

g_make_token_header() and g_token_size() add two too many, and
therefore their callers pass in "(logical_value - 2)" rather
than "logical_value" as hard-coded values which causes confusion.

This dates back to the original g_make_token_header which took an
optional token type (token_id) value and added it to the token.
This was removed, but the routine always adds room for the token_id
rather than not.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>