Stack overflow in recursive bpobj_iterate_impl

The function bpobj_iterate_impl overflows the stack when bpobjs
are deeply nested. Rewrite the function to eliminate the recursion.

Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Zuchowski <pzuchowski@datto.com>
Closes #7674
Closes #7675
Closes #7908