block: Convert bdrv_co_discard() to byte-based
Another step towards byte-based interfaces everywhere. Replace the sector-based bdrv_co_discard() with a new byte-based bdrv_co_pdiscard(), which silently ignores any unaligned head or tail. Driver callbacks will be converted in followup patches. By calculating the alignment outside of the loop, and clamping the max discard to an aligned value, we can simplify the actions done within the loop. Signed-off-by:Eric Blake <eblake@redhat.com> Reviewed-by:
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