btrfs-progs: mkfs: do zeroing before discard

mkfs.btrfs clears the very beginning and very end of a block device, to
prevent it from potentially being identified as another filesystem.

Change btrfs_prepare_device() so that this happens immediately before the
whole-device discard, rather than afterwards. This means that the contents
of these regions is either all zeroes (if discard did nothing) or indeterminate
(if discard was effective): either way it's clearing the other filesystem's
markers.

Pull-request: #1021
Signed-off-by: Mark Harmstone <mark@harmstone.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: maharmstone

common/device-utils.c

@@ -277,8 +277,6 @@ int btrfs_prepare_device(int fd, const char *file, u64 *byte_count_ret,
 				goto err;
 			}
 		}
-	} else if (opflags & PREP_DEVICE_DISCARD) {
-		prepare_discard_device(file, fd, byte_count, opflags);
 	}
 
 	ret = zero_dev_clamped(fd, zinfo, 0, ZERO_DEV_BYTES, byte_count);
@@ -295,6 +293,9 @@ int btrfs_prepare_device(int fd, const char *file, u64 *byte_count_ret,
 		goto err;
 	}
 
+	if (!(opflags & PREP_DEVICE_ZONED) && opflags & PREP_DEVICE_DISCARD)
+		prepare_discard_device(file, fd, byte_count, opflags);
+
 	ret = btrfs_wipe_existing_sb(fd, zinfo);
 	if (ret < 0) {
 		error("cannot wipe superblocks on %s", file);