rbd: Clear ceph_msg->bio_iter for retransmitted message

(cherry picked from commit 43643528cce60ca184fe8197efa8e8da7c89a037)
(cherry picked from commit b132cf4c733f91bb4dd2277ea049243cf16e8b66)

The bug can cause NULL pointer dereference in write_partial_msg_pages

Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
Reviewed-by: Alex Elder <elder@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>