x86/microcode/intel: Use a reserved field for metasize

Intel is using microcode file format for IFS test images too.

IFS test images use one of the existing reserved fields in microcode
header to indicate the size of the region in the file allocated for
metadata structures.

In preparation for this, rename first of the existing reserved fields
in microcode header to metasize. In subsequent patches IFS specific
code will make use of this field while parsing IFS images.

Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/r/20221117035935.4136738-10-jithu.joseph@intel.com