x86: Fix value for `x86_memset_non_temporal_threshold` when it is undesirable

x86: Fix value for `x86_memset_non_temporal_threshold` when it is undesirable

When we don't want to use non-temporal stores for memset, we set
`x86_memset_non_temporal_threshold` to SIZE_MAX.

The current code, however, we using `maximum_non_temporal_threshold`
as the upper bound which is `SIZE_MAX >> 4` so we ended up with a
value of `0`.

Fix is to just use `SIZE_MAX` as the upper bound for when setting the
tunable.
Tested-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>