bs_rbd.c: store active_rbd info in addition to struct scsi_lu

Use bs_datasize feature to allocate bs_rbd-private data on the end
of scsi_lu, and then access it with pointer arithmetic; no need to
manage an array indexed by fd, so no fixed number of devices anymore.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
bs_rbd.c: carry cluster connection along with rbd/lu

Allow each mapped image to also contain its own private cluster
connection.  This will allow one tgtd to serve multiple clients
with potentially different authentication rights.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>