dm dust: add limited write failure mode

Add a limited write failure mode which allows a write to a block to fail
a specified amount of times, prior to remapping.  The "addbadblock"
message is extended to allow specifying the limited number of times a
write fails.

Example: add bad block on block 60, with 5 write failures:
  dmsetup message 0 dust1 addbadblock 60 5

The write failure counter will be printed for newly added bad blocks.

Signed-off-by: Bryan Gurney <bgurney@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>