fusefs: support asynchronous cache invalidation

Protocol 7.12 adds a way for the server to notify the client that it should
invalidate an inode's data cache and/or attributes. This commit implements
that mechanism. Unlike Linux's implementation, ours requires that the file
system also supports FUSE_EXPORT_SUPPORT (NFS-style lookups). Otherwise the
invalidation operation will return EINVAL.

Sponsored by: The FreeBSD Foundation