fusefs: check the vnode cache when looking up files for the NFS server

FUSE allows entries to be cached for a limited amount of time. fusefs's
vnop_lookup method already implements that using the timeout functionality
of cache_lookup/cache_enter_time. However, lookups for the NFS server go
through a separate path: vfs_vget. That path can't use the same timeout
functionality because cache_lookup/cache_enter_time only work on pathnames,
whereas vfs_vget works by inode number.

This commit adds entry timeout information to the fuse vnode structure, and
checks it during vfs_vget. This allows the NFS server to take advantage of
cached entries. It's also the same path that FUSE's asynchronous cache
invalidation operations will use.

Sponsored by: The FreeBSD Foundation