amd64_edac: Improve error injection

When injecting DRAM ECC errors over the F3xB[8,C] interface, the machine
does this by injecting the error in the next non-cached access. This
takes relatively long time on a normal system so that in order for us to
expedite it, we disable the caches around the injection.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>