ext2fs: add dir_nlink feature support.

ext4 on linux has always supported more than 32000 directories through
the dir_nlink feature, but FreeBSD was unable to catch up on this feature.
As part of the 64 bit inode changes nlink_t has been extended and this
feature is now possible.

Submitted by: Fedor Uporov
Differential Revision: https://reviews.freebsd.org/D11210