Send IDLE IMMEDIATE for warm boot.

We must send either an IDLE IMMEDIATE or a STANDBY IMMEDIATE to drives
on warm boot so their SMART and other volatile data is
persisted. However, for a warm boot we don't want to send STANDBY
IMMEDIATE to some spinning drives because they will spin down. If
there's a lot of these drives on the system, that can cause a
thundering herd problem at startup time (that in extreme cases causes
timeout in device discovery).

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D12811