GFS2: When adding a new dir entry, inc link count if it is a subdir

This adds an increment of the link count when we add a new directory
entry, if that entry is itself a directory. This means that we no
longer need separate code to perform this operation.

Now that both adding and removing directory entries automatically
update the parent directory's link count if required, that makes
the code shorter and simpler than before.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>