nfs4: minor callback code simplification, comment

Note the position in the version array doesn't have to match the actual
rpc version number--to me it seems clearer to maintain the distinction.

Also document choice of rpc callback version number, as discussed in
e.g. http://www.ietf.org/mail-archive/web/nfsv4/current/msg07985.html


and followups.

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