panic(9): Serialize panicstr access and printing `panic:' message.

This isn't riskier than before -- previously we took kprintf_lock
inside each separate printf/vprintf call here.  Now we just take it
once around access to panicstr and printing the message.

With any luck, this should help avoid interleaving panic messages
with each other and with other output -- and maybe cut down on the
number of syzkaller duplicates.

(riastradh)