dm flakey: clone pages on write bio before corrupting them

dm flakey: clone pages on write bio before corrupting them

dm-flakey has an option to corrupt write bios. It corrupts the memory that
is being written. This can cause system crashes or security bugs - for
example, if the user writes a shared library code with O_DIRECT flag to a
dm-flakey device, it corrupts the memory for all users that have the
shared library mapped.

Fix this bug by cloning the bio and corrupting the clone rather than
the original.

Also drop the test for ZERO_PAGE(0) - it can't happen because we write
the cloned pages.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>