NFSv4.2: Clean up: move decode_*xattr() functions

Move them out of the encode_*() section and into the decode_*() section
where it belongs.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>