nfs: replace uses of __constant_{endian}

The base versions handle constant folding now, none of these headers
are exported to userspace, so the __ prefixed versions are not
necessary.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>