kern/88725: /usr/sbin/ppp panic with 2005.10.21 netinet6 changes

[Mark Tinguely]

As a postscript:

 The problem was a dynamic timer was freed without being stopped first.
 Obviously, the printf() should be removed from the final fix.

 After this discovery, I went through all of the callout_init() calls
 in the kernel and looked at those that may be freed before possibly
 being stopped. Beside the one in netinet6/mld6.c, I have 5 more
 that initially look like the memory for the callout struction could
 also be freed and still not have been stopped. These paths are problably
 not traveled much (detaches for less mainstream components), but stopping
 the callout is cheap and not at all risky.

 I will look at the 5 cases again and suggest all of these callout at
 risk be stopped under the same fix.

--Mark Tinguely