block: Fix integer promotion error in bdrv_getlength()

block: Fix integer promotion error in bdrv_getlength()

Back in 2015, we attempted to fix error reporting for images that
claimed to have more than INT64_MAX/512 sectors, but due to the type
promotions caused by BDRV_SECTOR_SIZE being unsigned, this
inadvertently forces all negative ret values to be slammed into -EFBIG
rather than the original error.  While we're at it, we can avoid the
confusing ?: by spelling the logic more directly.

Fixes: 4a9c9ea0d3
Reported-by: Guoyi Tu <tu.guoyi@h3c.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20201105155122.60943-1-eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>