librbd ioengine support

Initially only DDIR_WRITE/DDIR_READ and DDIR_SYNC are supported.
All other io_u->ddir get silently ignored.

A known issue requires to call the rbd engine always with --invalidate=0

Signed-off-by: Daniel Gollub <d.gollub@telekom.de>
Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
5 files changed