crypto: xor - defer load time benchmark to a later time

Currently, the XOR module performs its boot time benchmark at core
initcall time when it is built-in, to ensure that the RAID code can
make use of it when it is built-in as well.

Let's defer this to a later stage during the boot, to avoid impacting
the overall boot time of the system. Instead, just pick an arbitrary
implementation from the list, and use that as the preliminary default.

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>