x86/ucode/intel: Clean up microcode_update_match()

x86/ucode/intel: Clean up microcode_update_match()

Implement a new get_ext_sigtable() helper to abstract the logic for
identifying whether an extended signature table exists.  As part of this,
rename microcode_intel.bits to data and change its type so it can be usefully
used in combination with the datasize header field.

Also, replace the sigmatch() macro with a static inline with a more useful
API, and an explanation of why it is safe to drop one of the previous
conditionals.

No practical change in behaviour.

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