OpenZFS 9442 - decrease indirect block size of spacemaps

OpenZFS 9442 - decrease indirect block size of spacemaps

Authored by: Matthew Ahrens <mahrens@delphix.com>
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>
Reviewed by: George Melikov <mail@gmelikov.ru>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
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.

Porting notes:
* Refactored to allow the dmu_object_alloc(), dmu_object_alloc_ibs()
  and dmu_object_alloc_dnsize() functions to use a common shared
  dmu_object_alloc_impl() function.

OpenZFS-issue: https://www.illumos.org/issues/9442
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/0c2e6408b
Closes #7712