NFSv4.2: Clean up nfs4_xdr_dec_*xattr() functions

I add commends above each function to match the style of the other
nfs4_xdr_dec_*() functions. I also remove the unnecessary #ifdef
CONFIG_NFS_V4_2 that was added around this code, since we are already in
a v4.2-only file.

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