erofs: correct FSDAX detection

The detection of the primary device is skipped incorrectly
if the multiple or flattened feature is enabled.

It also fixes the FSDAX misdetection for non-block extra blobs.

Fixes: c6993c4 ("erofs: Fallback to normal access if DAX is not supported on extra device")
Reported-and-tested-by: syzbot+31b8fb02cb8a25bd5e78@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/691af9f6.a70a0220.3124cb.0097.GAE@google.com
Cc: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Author: hsiangkao

fs/erofs/super.c

@@ -174,15 +174,15 @@ static int erofs_init_device(struct erofs_buf *buf, struct super_block *sb,
 		if (!erofs_is_fileio_mode(sbi)) {
 			dif->dax_dev = fs_dax_get_by_bdev(file_bdev(file),
 					&dif->dax_part_off, NULL, NULL);
-			if (!dif->dax_dev && test_opt(&sbi->opt, DAX_ALWAYS)) {
-				erofs_info(sb, "DAX unsupported by %s. Turning off DAX.",
-					   dif->path);
-				clear_opt(&sbi->opt, DAX_ALWAYS);
-			}
 		} else if (!S_ISREG(file_inode(file)->i_mode)) {
 			fput(file);
 			return -EINVAL;
 		}
+		if (!dif->dax_dev && test_opt(&sbi->opt, DAX_ALWAYS)) {
+			erofs_info(sb, "DAX unsupported by %s. Turning off DAX.",
+				   dif->path);
+			clear_opt(&sbi->opt, DAX_ALWAYS);
+		}
 		dif->file = file;
 	}
 
@@ -215,13 +215,13 @@ static int erofs_scan_devices(struct super_block *sb,
 			  ondisk_extradevs, sbi->devs->extra_devices);
 		return -EINVAL;
 	}
-	if (!ondisk_extradevs) {
-		if (test_opt(&sbi->opt, DAX_ALWAYS) && !sbi->dif0.dax_dev) {
-			erofs_info(sb, "DAX unsupported by block device. Turning off DAX.");
-			clear_opt(&sbi->opt, DAX_ALWAYS);
-		}
-		return 0;
+
+	if (test_opt(&sbi->opt, DAX_ALWAYS) && !sbi->dif0.dax_dev) {
+		erofs_info(sb, "DAX unsupported by block device. Turning off DAX.");
+		clear_opt(&sbi->opt, DAX_ALWAYS);
 	}
+	if (!ondisk_extradevs)
+		return 0;
 
 	if (!sbi->devs->extra_devices && !erofs_is_fscache_mode(sb))
 		sbi->devs->flatdev = true;