MFV r272585:

Split the godfather zio into CPU number's to reduce lock
contention.

Illumos issue:
    5176 lock contention on godfather zio

MFC after:	2 weeks
MFV r272588:

Handle old format deadlist.

Illumos issue:
    5178 zdb -vvvvv on old-format pool fails in dump_deadlist()

MFC after:	2 weeks