Prior to this patch, a loop was used to issue multiple "fire and forget"
aios for each range in the command. Without a reference to the aiocb
returned from the blk_aio_pdiscard calls, the aios cannot be canceled.
Fix this by processing the ranges one after another.
As a bonus, this fixes how metadata is cleared (i.e. we only zero it out
if the data was succesfully discarded).
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
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