erasure-code: restore jerasure BlaumRoth default w

Changing from W=7 to W=6 by default for the BlaumRoth technique is
correct but introduces a regression. The content that was encoded with
the previous version cannot be read again. Although the prime(w+1)
constraint was not obeyed by W=7, the encoded content was useable and
should keep being readable.

The W=7 remains the default for backward compatibility and an exception
to the prime(w+1) check.

http://tracker.ceph.com/issues/9572 Fixes: #9572

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2556 from dachary/wip-9572-erasure-code-blaumroth

erasure-code: restore jerasure BlaumRoth default w

Reviewed-by: Sage Weil <sage@redhat.com>