dm-integrity: set ti->error on memory allocation failure

commit 00204ae3d6712ee053353920e3ce2b00c35ef75b upstream.

The dm-integrity target didn't set the error string when memory
allocation failed. This patch fixes it.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>