block/file-posix: Fix return value translation for AIO discards

block/file-posix: Fix return value translation for AIO discards

AIO discards regressed as a result of the following commit:
	0dfc7af2 block/file-posix: Optimize for macOS

When trying to run blkdiscard within a Linux guest, the request would
fail, with some errors in dmesg:

---- [ snip ] ----
[    4.010070] sd 2:0:0:0: [sda] tag#0 FAILED Result: hostbyte=DID_OK
driverbyte=DRIVER_SENSE
[    4.011061] sd 2:0:0:0: [sda] tag#0 Sense Key : Aborted Command
[current]
[    4.011061] sd 2:0:0:0: [sda] tag#0 Add. Sense: I/O process
terminated
[    4.011061] sd 2:0:0:0: [sda] tag#0 CDB: Unmap/Read sub-channel 42
00 00 00 00 00 00 00 18 00
[    4.011061] blk_update_request: I/O error, dev sda, sector 0
---- [ snip ] ----

This turns out to be a result of a flaw in changes to the error value
translation logic in handle_aiocb_discard(). The default return value
may be left untranslated in some configurations, and the wrong variable
is used in one translation.

Fix both issues.

Fixes: 0dfc7af2b28 ("block/file-posix: Optimize for macOS")
Cc: qemu-stable@nongnu.org
Signed-off-by: Ari Sundholm <ari@tuxera.com>
Signed-off-by: Emil Karlson <jkarlson@tuxera.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20211019110954.4170931-1-ari@tuxera.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>