Add support for the DIR_NLINK feature.

Add support for the DIR_NLINK feature.

This patch includes the changes required to e2fsck to understand the
nlink count changes made in the kernel.

In e2fsck pass 4, when we fetch the actual link count, if it is
exceeds 65,000 we set the link count to 1.  We silently fix the
situation where the nlink count of the directory is 1, and there are
fewer than 65,000 subdirectories, since since that can happen
naturally.

Patch originally from CFS, significantly rewritten by Theodore Ts'o.

Signed-off-by: Andreas Dilger <adilger@clusterfs.com>
Signed-off-by: Kalpak Shah <kalpak@clusterfs.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>