ext2fs: check for eh_depth in ext4_ext_check_header()

[nc]

Summary

PR: 259112
Reported by: Robert Morris <[e-mail]>

[fsu]

I am working on more complex extents verification logic, but let it to be committed.

Also:
https://github.com/torvalds/linux/blob/master/fs/ext4/extents.c#L467

	if (unlikely(depth > 32)) {
		error_msg = "too large eh_depth";
		goto corrupted;
	}