loader: update symlink support in zfs reader

As the current zfs file system is providing symlink via system attributes, need
to update the code accordingly.

Note, as the zfsboot code does not free the memory at this time, the
object list will put some stress on the boot2 heap, eventually we should
address the issue.

Reviewed by: allanjude, smh
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D9706