fusefs: raise protocol level to 7.12
This commit raises the protocol level and adds backwards-compatibility code
to handle structure size changes. It doesn't implement any new features.
The new features added in protocol 7.12 are:
- server-side umask processing (which FreeBSD won't do)
- asynchronous inode and directory entry invalidation (which I'll do next)
Sponsored by: The FreeBSD Foundation