xen, blkfront: do not allocate per-request private data

This commit fixes cmd_size size setting it to zero, as the driver
still doesn't use per-request private data.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
xen, blkfront: fix initialization of queue depth

The indirect segments feature increases the number of I/O in flight,
but does not increase the number of in-flight requests, which is
still BLK_RING_SIZE.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
xen, blkfront: fix locking of queue_rq hook

This fixes a bug that could cause panics when concurrently accessing
request-related ring fields.

Reported-by: Bob Liu <bob.liu@oracle.com>
Tested-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
xen, blkfront: separate request and response rings

This commit is the counterpart of blkback's b6e6832. This still leaves
locking intact and is not backwards-compatible (which means that both
the frontend and the backend need to be changed).
xen, blkfront: make req_cons visible to the front ring

This should allow to check for a full ring without reading from
rsp_* ring fields.