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>
diff --git a/options.c b/options.c
index 4dcefba..ea51664 100644
--- a/options.c
+++ b/options.c
@@ -1388,6 +1388,11 @@
 			    .help = "Windows native asynchronous IO"
 			  },
 #endif
+#ifdef CONFIG_RBD
+			  { .ival = "rbd",
+			    .help = "Rados Block Device asynchronous IO"
+			  },
+#endif
 			  { .ival = "mmap",
 			    .help = "Memory mapped IO"
 			  },