8061 sa_find_idx_tab can be declared more type-safely

illumos/illumos-gate@7f0bdb4257bb4f1f76390b72665961e411da24c6
https://github.com/illumos/illumos-gate/commit/7f0bdb4257bb4f1f76390b72665961e411da24c6

https://www.illumos.org/issues/8061

sa_find_idx_tab() is declared as taking and returning "void *" parameters.
These can be declared to be the specific types.

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Chris Williamson <chris.williamson@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Matthew Ahrens <mahrens@delphix.com>