erofs: correct FSDAX detection

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: c6993c4cb918 ("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>