rbd-nbd: check /sys/block/nbdX/size to ensure kernel mapped correctly

rbd-nbd: check /sys/block/nbdX/size to ensure kernel mapped correctly

Fixes: http://tracker.ceph.com/issues/18335
Signed-off-by: Mykola Golub <mgolub@mirantis.com>
(cherry picked from commit 596e5ea8a5df72002672eef0a6d20572ca6f60f0)

Conflicts:
	qa/workunits/rbd/rbd-nbd.sh: the original commit removes
	  TOO_LARGE_IMAGE test, do the same thing
	src/tools/rbd_nbd/rbd-nbd.cc: help git to add
	  "include/stringify.h"

Other changes:
	src/tools/rbd_nbd/rbd-nbd.cc: #include <fstream> so
	  the code compiles

Signed-off-by: Alexey Sheplyakov <asheplyakov@mirantis.com>