MFV r337167: 9442 decrease indirect block size of spacemaps

MFV r337167: 9442 decrease indirect block size of spacemaps

Updates to indirect blocks of spacemaps can contribute significantly to
write inflation.  Therefore we want to reduce the indirect block size of
spacemaps from 128K to 16K.

illumos/illumos-gate@221813c13b43ef48330b03725e00edee85108cf1

Reviewed by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Albert Lee <trisk@forkgnu.org>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Approved by: Dan McDonald <danmcd@joyent.com>
Author:     Matthew Ahrens <mahrens@delphix.com>