shrink_dentry_list(): no need to check that dentry refcount is marked dead

... we won't see DCACHE_MAY_FREE on anything that is *not* dead
and checking d_flags is just as cheap as checking refcount.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>