fsck: copy out device argument from argv[] before forking

fsck: copy out device argument from argv[] before forking

We nowadays rename our child processes, hence argv[] will be clobbered,
let's hence copy the device path to dynamic memory before forking.

This is fall-out from 60ffa37a65a96c3af857a3dfc4a6fd47b20cc90e since we
now a lot more often end up overriding the argv[] buffer than before,
simple because we know what to override.

These kind of bugs kinda suck. THere are only two options here: stop
overriding argv[] for all cases (or just these cases) or explicitly
copying out everything we need in child processes before forking. With
this patch I opt for the latter, though I am not 100% convinced this is
a great solution. Just a better solution than everything else, i.e.
allowing argv[] to remain out of sync with what others see.

Fixes: #12135