fix kABI breakage from "blk-mq: fix race between timeout and freeing request"

fix kABI breakage from "blk-mq: fix race between timeout and freeing request"

Orabug: 23521058

Commit 21d62f6 ("blk-mq: fix race between timeout and freeing request")
added the element "orig_rq" to struct blk_flush_queue.  This broke kABI.

The existing blk_flush_queue structure has "holes" (padding) of 4 bytes
plus 29 bits.  This is not enough space to contain orig_rq, which is 8
bytes long.

Currently, this data structure is only used by the block-device code.
It is unlikely that a third-party module (e.g., a device driver) would
use struct blk_flush_queue, and even if one did, it would most likely
be via a pointer.

This commit moves orig_rq to the end of the structure, so that the offsets
of the existing elements remain the same as before.  It also wraps orig_rq
in "#ifndef __GENKSYMS__" to hide it from the kABI checker.

Signed-off-by: Dan Duval <dan.duval@oracle.com>