x86: Split cache_flush() out of cache_writeback()

x86: Split cache_flush() out of cache_writeback()

Subsequent changes will want a fully flushing version.

Use the new helper rather than opencoding it in flush_area_local().  This
resolves an outstanding issue where the conditional sfence is on the wrong
side of the clflushopt loop.  clflushopt is ordered with respect to older
stores, not to younger stores.

Rename gnttab_cache_flush()'s helper to avoid colliding in name.
grant_table.c can see the prototype from cache.h so the build fails
otherwise.

This is part of XSA-402.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>