Avoid completing I/O when dumping core after a panic.

Filesystem or pager completion callbacks are generally non-functional
after a panic and may trigger deadlocks if invoked in this context
(e.g., by attempting to destroying a buffer mapping). To avoid this
situation, short-circuit I/O completion in biodone().

Reviewed by: imp
Discussed with: mav
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D15592