udev-block-add-change: better mount status check

The canonical device name is not necessarily used in /proc/mounts or
/proc/self/mountinfo, see /dev/dm-0 vs. /dev/mapper/dmroot. (And only
checking by major:minor is not possible in the case of btrfs.)

Also fixes another bug where e.g. dm-10 would match when really only
dm-1 was mounted, or when the mountpoint path included the device name,
etc.

Also catches enabled swap.

udev/udev-block-add-change

@@ -20,8 +20,8 @@ xs_remove() {
 
 is_used() {
     local sys_devpath=$1
-    # mounted
-    if fgrep -q $(basename $sys_devpath) /proc/mounts; then
+    # mounted; or enabled swap
+    if lsblk -dnr -o MOUNTPOINT "$DEVNAME" | grep -q .; then
         return 0
     fi
     # part of other device-mapper