Correct blkback handling of the BLKIF_OP_FLUSH_DISKCACHE opcode.
Correct blkback handling of the BLKIF_OP_FLUSH_DISKCACHE opcode.

Properly round-trip the "operation code" for client requests.

sys/dev/xen/blkback/blkback.c:
	In xbb_dispatch_dev() when processing a flush request,
	correctly set bio->bio_caller1 to the request list (not
	bare request) for the operation, as is expected by the
	completion handler xbb_bio_done().

	In xbb_get_resources(), initialize "operation" in the
	driver's internal request object from the client's "ring
	request", so it is correct when used to populate the reply
	when this operation completes.

Submitted by:	Roger Pau Monné
Sponsored by:	Citrix Systems R&D
Reviewed by:	gibbs

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