erasure-code: remove jerasure internals dependencies


Use the galois_init_default_field() function instead of implementing it
in the init function of the ErasureCodeJerasure plugin. It
removes dependencies to internal variables of the jerasure library that
are subject to change because they are not part of the published API.

http://tracker.ceph.com/issues/8071 Fixes: #8071

Signed-off-by: Loic Dachary <loic@dachary.org>