NFSD: change LISTXATTRS cookie encoding to big-endian

Function nfsd4_listxattr_validate_cookie() expects the cookie
as an offset to the list thus it needs to be encoded in big-endian.

Fixes: 23e50fe3

 ("nfsd: implement the xattr functions and en/decode logic")
Signed-off-by: Jorge Mora <mora@netapp.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>