CIFS: fix automount for DFS shares

Automounting directories are now invalidated by .d_revalidate()
so to be d_instantiate()d again with the right DCACHE_NEED_AUTOMOUNT
flag

Reviewed-by: Jeff Layton <jlayton@samba.org>
Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Signed-off-by: Steve French <smfrench@gmail.com>