ForkFilter.java: Fix issue when reading files with multiple extents.

We did not properly seek to the correct location after moving to a new
extent, meaning that the wrong data was returned.
Move HFS+ specific link functionality out of hfscommon storage package.

If it doesn't apply to both HFS and HFS+, it shouldn't be in this
package. The code was moved to HFSPlusFileSystemHandler in hfsplus
package.