generic/085: general cleanup for reliability and debugging

generic/085: general cleanup for reliability and debugging

This test was quite unreliable during development of the parallel
check runner. It redirects all errors to /dev/null, so there was no
way to debug it when it failed.

Use common mount/unmount helpers, redirect errors to $seqres.full,
make sure the cleanup code is always run at test exit and only
attempt to kill processes if they are still running during cleanup.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Zorro lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>