osd/osd_types: bump encoding from 11 -> 12

Meant to do this in a1b82f2a568ba211601c12b0b33474e982a3be6f or
d421b66293fb3d815ad2cd4c787dd2c39f48d6e8 but forgot!

Signed-off-by: Sage Weil <sage@inktank.com>