Add root directory entry check.

[fsu]

Summary

Add check that directory entry with ino=EXT2_ROOTINO have correct namelength and name. It is possible to create malicious image which will cause panic if root directory entry have incorrect name.

Fix: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=259068

Test Plan

ext2fs does not panic on malicious image from 259068

[pfg]

I think this is the first time I have seen a char string return value. Can we keep the same coding style as in ext2_check_direntry() ?
That is .. assigning error_msg within the function instead of returning it.

[fsu]

Ok, let's remove ext2_check_root_direntry() completely.

[pfg]

LGTM