file-posix: account discard operations

file-posix: account discard operations

This will help to identify how many of the user-issued discard operations
(accounted on a device level) have actually suceeded down on the host file
(even though the numbers will not be exactly the same if non-raw format
driver is used (e.g. qcow2 sending metadata discards)).

Note that these numbers will not include discards triggered by
write-zeroes + MAY_UNMAP calls.

Signed-off-by: Anton Nefedov <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-id: 20190923121737[email protected]
Signed-off-by: Max Reitz <[email protected]>