Findmnt returns 1 when no matching entries found

Our local fence script attempts to interpret errors executing `findmnt`
as critical errors, but the program exit code explicitly returns
EXIT_FAILURE when the total number of matching mount entries is zero.

This can happen if the mount disappeared while we're attempting to
fence the mount, but, the scoutfs sysfs files are still in place as
we read them. It's a small window, but, it's a fork/exec plus full
parse of /etc/fstab, and a lot can happen in the 0.015s findmnt takes
on my system.

There's no other exit codes from findmnt other than 0 and 1. At that
point, we can only assume that if the stdout is empty, the mount
isn't there anymore.

Signed-off-by: Auke Kok <auke.kok@versity.com>

tests/fenced-local-force-unmount.sh

@@ -27,8 +27,7 @@ for fs in /sys/fs/scoutfs/*; do
 	nr="$(quiet_cat $fs/data_device_maj_min)"
 	[ ! -d "$fs" -o "$fs_rid" != "$rid" ] && continue
 
-	mnt=$(findmnt -l -n -t scoutfs -o TARGET -S $nr) || \
-		echo_fail "findmnt -t scoutfs -S $nr failed"
+	mnt=$(findmnt -l -n -t scoutfs -o TARGET -S $nr)
 	[ -z "$mnt" ] && continue
 
 	if ! umount -qf "$mnt"; then