NFSD: Combine decode operations for v4 and v4.1

We were using a different array of function pointers to represent each
minor version.  This makes adding a new minor version tedious, since it
needs a step to copy, paste and modify a new version of the same
functions.

This patch combines the v4 and v4.1 arrays into a single instance and
will check minor version support inside each decoder function.

Signed-off-by: Anna Schumaker <bjschuma@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>