Add more accurate directory entries check

Rename ext2_dirbadentry() to ext2_check_direntry(). Add directory
entry inode value check, and call ext2_check_direntry() in all cases.
The dirchk sysctl is removed.

PR: 259024,259041
Reported by: Robert Morris
Reviewed by: pfg
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33374