Use variable and shorter logic for NFS check

Use variable and shorter logic for NFS check

No need to hardcode a parent path if we can reuse an existing variable for
the specific path that is in being used.

Negate the file system type in df(1) so the `|| exit 1' can be dropped
in favour of the errexit option, as is done for everything else in there.

Clarify the comment how this is intentionally NOT logged, i.e. the test
happens before the error trap/syslog/logfile handling.

OK millert