erofs: handle NONHEAD !delta[1] lclusters gracefully

In the Linux kernel, the following vulnerability has been resolved:

erofs: handle NONHEAD !delta[1] lclusters gracefully

syzbot reported a WARNING in iomap_iter_done:

iomap_fiemap+0x73b/0x9b0 fs/iomap/fiemap.c:80

ioctl_fiemap fs/ioctl.c:220 [inline]

Generally, NONHEAD lclusters won't have delta[1]==0, except for crafted

images and filesystems created by pre-1.0 mkfs versions.

Previously, it would immediately bail out if delta[1]==0, which led to

inadequate decompressed lengths (thus FIEMAP is impacted). Treat it as

delta[1]=1 to work around these legacy mkfs versions.

`lclusterbits > 14` is illegal for compact indexes, error out too.