ZTS: Fix stale symlinks with zfs-helpers.sh

zfs-helpers.sh is a utility script that sets up udev symlinks so you
can run ZTS from a local ZFS git workspace.  However, it doesn't check
that the udev symlinks point to the current workspace.  They may point
to an old workspace that has been deleted.  This means the udev rules
never get executed, which in turn causes the zvol tests to fail.

This commit removes old symlinks that do not point to the current
ZFS workspace.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Tony Hutter <[email protected]>
Closes #17766

Author: tonyhutter

scripts/zfs-helpers.sh

@@ -122,6 +122,13 @@ install() {
 	src=$1
 	dst=$2
 
+	# We may have an old symlink pointing to different ZFS workspace.
+	# Remove the old symlink if it doesn't point to our workspace.
+	if [ -h "$dst" ] && [ "$(readlink -f """$dst""")" != "$src" ] ; then
+		echo "Removing old symlink: $dst -> $(readlink """$dst""")"
+		rm "$dst"
+	fi
+
 	if [ -h "$dst" ]; then
 		echo "Symlink exists: $dst"
 	elif [ -e "$dst" ]; then