NFS: Remove asserts from the NFS XDR code

Convert the ones that are not trivial to check into WARN_ON_ONCE().
Remove checks for things such as NFS2_MAXPATHLEN, which are trivially
done by the caller.

Add a comment to the case of nfs3_xdr_enc_setacl3args. What is being
done there is just wrong...

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