Revert to use umount -l in storage tests cleanup. With fixed permissions
in 4234fe51 "tests: fix cleanup after reflink tests", it shouldn't cause
issues anymore, but apparently on some systems test cleanup fails
otherwise.
Reported by @rustybird
This reverts commit b6f77ebfa1.
If unmount is going to fail, let it do so explicitly, instead of hiding
the failure now, and observing it later at rmdir.
And if it fails, lets report what process is using that mount point.
