erasure-code: jerasure preloads the plugin variant

erasure-code: jerasure preloads the plugin variant

The variant selection depending on the available CPU features is
encapsulated in a helper. The helper is used in the factory() method and
in the load() method.

The factory() method may load a variant that is not the default, for
benchmark purposes. Such a variant is not preloaded by the load() method
and upgrading while running may be problematic. However, running with a
non standard variant is used for benchmarking and upgrades in this
context are not a concern.

http://tracker.ceph.com/issues/9153 Fixes: #9153

Signed-off-by: Loic Dachary <loic-201408@dachary.org>