fusefs: raise protocol level to 7.12

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