OpenZFS 7054 - dmu_tx_hold_t should use refcount_t to track space

Authored by: Igor Kozhukhov ikozhukhov@gmail.com
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>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov mail@gmelikov.ru

OpenZFS-issue: https://www.illumos.org/issues/7054
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/0c779ad
Closes #5600