Instead of using timestamp in the AVL, use the memory address when
comparing.

Illumos issue:
    5095 panic when adding a duplicate dbuf to dn_dbufs

MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270383 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f