MFC r250900:
Fix NULL-dereference kernel panic in case of mps_attach() failure.
MFC r250819:
Fix vdc->Secondary_Element_Count metadata field access from 16 to 8 bit.
In some cases it could cause kernel panic during failed drive replacement.
MFC r250508:
Disable sending Early R_OK on SiI3726/SiI3826 port multipliers.
With "cached read" HDD testing and multiple ports busy on a SATA
host controller, 3726/3826 PMP will very rarely drop a deferred
R_OK that was intended for the host. Symptom will be all 5 drives
under test will timeout, get reset, and recover.