OpenZFS 7500 - Simplify dbuf_free_range by removing dn_unlisted_l0_blkid

Authored by: Stephen Blinick <stephen.blinick@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Approved by: Gordon Ross <gordon.w.ross@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7500
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/653af1b
Closes #5639