Reviewed code in dentry.c and {read,write}_metadata_resource()

Reviewed code in dentry.c and {read,write}_metadata_resource()

- Added lots of comments.
- Fix writing the metadata resource when there is security data with length that
  isn't a multiple of 8 bytes.
- write_dentry_tree() is now a wrapper around write_dentry_tree_recursive(), the
  latter of which does not need to treat the root dentry specially.