erasure-code: thread-safe initialization of gf-complete

Instead of relying on an implicit initialization happening during
encoding/decoding with galois.c:galois_init_default_field, call
gf.c:gf_init_easy for each w values when the plugin is loaded.

Loading the plugin is protected against race conditions by a lock.

It does not cover all possible uses of gf-complete but it is enough for
the ceph jerasure plugin.

http://tracker.ceph.com/issues/7914 fixes #7914

Signed-off-by: Loic Dachary <loic@dachary.org>