7054 dmu_tx_hold_t should use refcount_t to track space

illumos/illumos-gate@0c779ad424a92a84d1e07d47cab7f8009189202b
https://github.com/illumos/illumos-gate/commit/0c779ad424a92a84d1e07d47cab7f8009189202b

https://www.illumos.org/issues/7054

upstream:
ee0003de7d3e598499be7ac3fe6b61efcc47cb7f
DLPX-40399 dmu_tx_hold_t should use refcount_t to track space

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Matthew Ahrens <mahrens@delphix.com>