NFS: Make callbacks minor version generic

I found a few places that hardcode the minor version number rather than
making it dependent on the protocol the callback came in over.  This
patch makes it easier to add new minor versions in the future.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>