[Toybox] mount option to show what a loopback mount is backed by?

[enh]

mount currently shows something like:

/dev/block/loop86 on /apex/com.android.hardware.tetheroffload at 1 type ext4
(ro,dirsync,seclabel,nodev,noatime)

but often the user wants to know what "loop86" refers to. and it's unlikely
they know to look in /sys/block/loop*/loop/backing_file.

afaik, though, there's no coreutils option to show this, so it would
involve making up a new command-line option as well as some new output...
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