fusefs: updated cached attributes during VOP_LINK.

FUSE_LINK returns a new set of attributes. fusefs should cache them just
like it does during other VOPs. This is not only a matter of performance
but of correctness too; without caching the new attributes the vnode's nlink
value would be out-of-date.

Reported by: pjdfstest
Sponsored by: The FreeBSD Foundation