vfs: reinstate in-use check in d_invalidate

FUSE userspace such as ceph-fuse relied on this
behaviour in 3.17 and earlier, in order to achieve
explicit kernel cache invalidation.  This explicit
invalidation is important in clients for
network filesystems that may have to release resources
in response to commands from a server.

Signed-off-by: John Spray <john.spray@redhat.com>