Revert changes to zbookmark_t

Commit 1421c89142376bfd41e4de22ed7c7846b9e41f95 added a field to
zbookmark_t that unintentinoally caused a disk format change. This
negatively affected backward compatibility and platform portability.
Therefore, this field is being removed.

The function that field permitted is left unimplemented until a later
patch that will reimplement the field in a way that does not affect the
disk format.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2094