ANDROID: uapi: Add dm-user structure definition

dm-user is a device mapper target that allows a userspace process to
handle each incoming BIO.  Communication with userspace consists of a
stream of messages proxied over a misc device, the structure of each
message is defined in this header.

Test: none
Bug: 161496058
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Change-Id: I97ca538fbd7e73e467416590190d85894db42e7d