MFC r249468:
Stop abusing xpt_periph in random plases that really have no periph related
to CCB, for example, bus scanning.  NULL value is fine in such cases and it
is correctly logged in debug messages as "noperiph".  If at some point we
need some real XPT periphs (alike to pmpX now), quite likely they will be
per-bus, and not a single global instance as xpt_periph now.
MFC r252569:
Fix NULL-deference kernel panic on attempt of destroying non-existing
ramdisk-backed CTL LUN.
MFC r252204, r252250:
Add bunch of names for Seagate and HGST vennor-specififc ASC/ASCQ codes.