MFC r272511: MFV r272499:

Illumos issue:
    5174 add sdt probe for blocked read in dbuf_read()
MFC r272584: MFV r272501:

Illumos issue:
    5177 remove dead code from dsl_scan.c
MFC r272598: MFV r272585:

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

Illumos issue:
    5176 lock contention on godfather zio
MFC r272599: MFV r272588:

Handle old format deadlist.

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