fstests: per-test dmflakey instances

fstests: per-test dmflakey instances

We can't run two tests that use dmflakey at the same time because
the device name is the same. hence they interfere with each other.
Given dmflakey devices their own per-test names to avoid this
problem.

Also, drop_and_remount is about to fail the fs during unmount, so
ensure the filesystem is going to fail the IO during unmount rather
than retrying forever.

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