Tinker with slop space accounting with dedup

• Tinker with slop space accounting with dedup

Do not include the deduplicated space usage in the slop space
reservation, it leads to surprising outcomes.

• Update spa_dedup_dspace sometimes

Sometimes, we get into spa_get_slop_space() with
spa_dedup_dspace=~0ULL, AKA "unset", while spa_dspace is correctly set.

So call the code to update it before we use it if we hit that case.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12271