dm dust: change ret to r in dust_map_read and dust_map

In the dust_map_read() and dust_map() functions, change the
return code variable "ret" to "r", to match the convention of the
other device-mapper targets.

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