rbd: get/put img_request in rbd_img_request_submit()

By the time we get to checking for_each_obj_request_safe(img_request)
terminating condition, all obj_requests may be complete and img_request
ref, that rbd_img_request_submit() takes away from its caller, may be
put.  Moving the next_obj_request cursor is then a use-after-free on
img_request.

It's totally benign, as the value that's read is never used, but
I think it's still worth fixing.

Cc: Alex Elder <elder@linaro.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>