Fix umount.nfs exit status

Hi,

As per a bug report from a user:

mount.c seems to assume that nfsumount() uses standard C true/false
return values, and inverts them for the exit status (where 0 is
traditionally considered success). However, nfsumount() consistently
seems to use 0 for success, and thus a success gets returned as exit
status 1 and a failure as exit status 0. This confuses at least
the GNOME drive manager applet, and probably others as well.

Signed-off-by: Steinar H. Gunderson <sesse@debian.org>
Signed-off-by: Neil Brown <neilb@suse.de>