mount: fix removing mount point if it's expired

Kernel can report mount point as expired and when that happens mountd
unmounts it. When such a device disappears it's still important to:
1) Cleanup directories
2) Call hotplug scripts
Fix this by adding a new EXPIRED status and checking it when block
device disappears.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>