ip6mrouter: Make the expiration callout MPSAFE

• 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.

Reported by: mav
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31836