fscache_cookie_enabled: check cookie is valid before accessing it

commit 0dc54bd4 upstream.

fscache_cookie_enabled() could be called on NULL cookies and cause a
null pointer dereference when accessing cookie flags: just make sure
the cookie is valid first

Suggested-by: David Howells <dhowells@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Cc: Jeffrey E Altman <jaltman@auristor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>