nfsd4: separate splice and readv cases

The splice and readv cases are actually quite different--for example the
former case ignores the array of vectors we build up for the latter.

It is probably clearer to separate the two cases entirely.

There's some code duplication between the split out encoders, but this
is only temporary and will be fixed by a later patch.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>