Illumos 3693 - restore_object uses at least two transactions to restore an object

restore_object uses at least two transactions to restore an object:

  * one transaction is used for dmu_object_claim
  * another transaction is used to set compression, checksum and most
    importantly bonus data
  * furthermore dmu_object_reclaim internally uses multiple transactions
  * dmu_free_long_range frees chunks in separate transactions
  * dnode_reallocate is executed in a distinct transaction

The fact the dnode_allocate/dnode_reallocate are executed in one
transaction and bonus (re-)population is executed in a different
transaction may lead to violation of ZFS consistency assertions if the
transactions are assigned to different transaction groups.  Also, if
the first transaction group is successfully written to a permanent
storage, but the second transaction is lost, then an invalid dnode may
be created on the stable storage.

Reviewed by: Christopher Siden christopher.siden@delphix.com
Reviewed by: George Wilson george.wilson@delphix.com

Original authors: Matthew Ahrens and Andriy Gapon

References:
  https://www.illumos.org/projects/illumos-gate//issues/3693
  https://reviews.csiden.org/r/90

Ported by: Turbo Fredriksson <turbo@bayour.com>