ide: fix memory leak when flush command is issued

I'm using ide on 2.6.30.1 with xfs filesystem. I noticed a kernel memory
leak after writing lots of data, the kmalloc-96 slab cache keeps
growing. It seems the struct ide_cmd kmalloced by idedisk_prepare_flush
is never kfreed.

Commit a09485df ("ide: move request
type specific code from ide_end_drive_cmd() to callers (v3)") and
f505d49f

 ("ide: fix barriers support")
cause this regression, cmd->rq must now be set for ide_complete_cmd to
honor the IDE_TFLAG_DYN flag.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Acked-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>