erasure-code: fix BlaumRoth sanity check on w

erasure-code: fix BlaumRoth sanity check on w

For the BlaumRoth technique, w+1 must be a prime. This sanity check is
present in the sample code from the jerasure library but was not
implemented in the jerasure plugin.

The default value for w is also changed to match this constraint. The
unit tests is changed to always use the default w instead of forcing it
to w.

http://tracker.ceph.com/issues/6754 Fixes: #6754

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