ip6mrouter: Make the expiration callout MPSAFE

[markj]

Summary

- Protect the `expire_upcalls` callout with the MFC6 mutex.  The callout
  handler needs this mutex anyway.
- Convert the MROUTER6 mutex to a sleepable sx lock.  It is only used
  when configuring the global v6 multicast routing socket, so is only
  used in system call paths where sleeping is safe.  This lets us drain
  the callout without having to drop the lock.
- For all locking macros in the file, convert to using a _LOCKPTR macro.

Test Plan

Just build-tested so far, some basic test program is needed.

[mav]

sys/netinet6/ip6_mroute.c

584

What's the point of locking it here?

[markj]

sys/netinet6/ip6_mroute.c

584

I forgot that the lock is not required by callout_reset().

[markj]

• Remove unneeded locking.
• Remove stray whitespace change.