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>