rbd: still racing

A multi-object request could still have two concurrent completions
that both think they need to clean up the image request.  The lock
protecting the test of next == count isn't enough.

Instead, have rbd_block_request_complete() return true iff this
particular call was the one that caused the completion.  Make
use of the return value of blk_end_request for this.

Signed-off-by: Alex Elder <elder@inktank.com>