Add root directory entry check.

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.

PR: 259068
Reported by: Robert Morris
Reviewed by: pfg
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D38502