man: move zfs_prepare_disk.8 to nodist_man_MANS

The commit b53077a added zfs_prepare_disk.8 to the wrong list
dist_man_MANS, in which @zfsexecdir@ will not be properly substituted.
This leads to wrong path in the manpage in generated release tarballs.

Reported-by: Benda Xu <orv@debian.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Shengqi Chen <harry-chen@outlook.com>
Closes #15979