erasure-code: preload the jerasure plugin variant (sse4,sse3,generic)

The preloading of the jerasure plugin ldopen the plugin that is in
charge of selecting the variant optimized for the
CPU (sse4,sse3,generic). The variant plugin itself is not loaded because
it does not happen at load() but when the factory() method is called.

The JerasurePlugin::preload method is modified to call the factory()
method to load jerasure_sse4 or jerasure_sse3 or jerasure_generic as a
side effect.

Indirectly loading another plugin in the factory() method is error prone
and should be moved to the load() method instead. This change should be
done in a separate commit.

http://tracker.ceph.com/issues/9153 Fixes: #9153

Signed-off-by: Loic Dachary <loic-201408@dachary.org>