erasure-code: refactor jerasure Liberation parameters checks

Create a virtual function to check for each parameter for Liberation
and its derivations Liber8tion and BlaumRoth. The code is moved around
but is unmodified.

A function reseting the parameters to sane defaults when an error
occurs is introduced. It is slightly different from the code that was
duplicated in Liberation and BlaumRoth. In both cases it now resets w.
It is a noop for Liber8tion because the user cannot modify this
parameter which is always set to the default.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
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>
Merge pull request #2442 from dachary/wip-6754-jerasure-parameters

erasure-code: fix BlaumRoth sanity check on w

Reviewed-by: Andreas Peters <andreas.joachim.peters@cern.ch>