This function should return either 0 or -EINSUFFFRAGS and can be used
as a quick check ahead of actually attempting reconstruction. If not
provided, default to checking for at least k fragments.
Implement the new function for flat xor codes so they can reconstruct
in more cases.
Change-Id: Ifa373e4ef8ef3caedb709c40c2b2bfd6fdf6ff7e
Signed-off-by: Tim Burke <tim.burke@gmail.com>