erasure-code: do not preload the isa plugin

Because it's not built for all architectures and distributions.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2295 from dachary/wip-9153-jerasure-upgrade

erasure-code: do not preload the isa plugin

Reviewed-by: Sage Weil <sage@redhat.com>