[Toybox] Comment on umount twiddling.

[Rob Landley]

I did the umount patch (always looking up mount points in /proc/mounts
so we can losetup -d the block device) because loopback mounts were
leaking loopback devices when I checked. But now I retest, the umount
syscall seems to be dissocating them...?

Possibly this is because this round of testing is on my ubuntu host and
my previous round was under aboriginal linux? (Possibly there's a kernel
config difference or ubuntu has a local patch?)

I committed the change anyway, because it doesn't hurt things when it's
not needed and sometimes it is needed. But it would be nice to
understand when that is...

Rob