erasure-code: use cauchy if K and M values are not supported by ISA-L reed_sol_van

erasure-code: use cauchy if K and M values are not supported by ISA-L reed_sol_van

ISA-L supports a limited set of K and M values when using a vandermonde matrix. There are no such limitations when using a cauchy matrix. If the user specifies reed_sol_van (or does not specify a technique and relies on the default reed_sol_van setting) and an unsupported K/M combination, then we will automatically switch the technique for the new EC profile to cauchy. Benchmarking has not shown any noticeable performance differences between ISA-L in reed_sol_van vs cauchy modes.

Signed-off-by: Jamie Pryde <jamiepry@uk.ibm.com>