Use KERNEL_PANICKED() in more places

This is slightly more optimized than checking panicstr directly. For
most of these instances performance doesn't matter, but let's make
KERNEL_PANICKED() the common idiom.

Reviewed by: mjg
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D35373

(cherry picked from commit 35eb9b10c265a27ce1f80a6eb74887240c7f4305)