umount.nfs: some umount.nfs error return codes are confusing

umount.nfs should return the standard EX_ mount return codes.  At some
point in the past, it was returning 0 for failure and 1 for success, and
some of these have been changed and some haven't.  See if we can rectify
this.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>