mount: create not working symlink when unmounting fails

mount: create not working symlink when unmounting fails

If device disappears but there is still some application trying to
access it, unmounting will fail with the EBUSY errno. Unlinking
/tmp/mounts/<symlink> would succeed but things could go wrong depending
on the application accessing that path.

If some app doesn't check if /tmp/mounts/<symlink> exists it could just
create it and start writing to it filling RAM instead of saving data to
a mounted drive.

Moreover the next time mountd tries to create that symlink it will fail
because there will be already a directory under that path. Things could
go even worse once mountd fires /sbin/hotplug-call and other apps start
using /tmp/mounts/<symlink> believing there's a mounteg storage there.

To fix this, detect failed umount call and replace /tmp/mounts/<symlink>
with a new symlink pointing to the not existing file. That will prevent
applications from re-creating that directory and writing to it filling
the RAM.

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