x86/ucode: Further fixes to identify "ucode already up to date"

x86/ucode: Further fixes to identify "ucode already up to date"

When the revision in hardware is newer than anything Xen has to hand,
'microcode_cache' isn't set up.  Then, `xen-ucode` initiates the update
because it doesn't know whether the revisions across the system are symmetric
or not.  This involves the patch getting all the way into the
apply_microcode() hooks before being found to be too old.

This is all a giant mess and needs an overhaul, but in the short term simply
adjust the apply_microcode() to return -EEXIST.

Also, unconditionally print the preexisting microcode revision on boot.  It's
relevant information which is otherwise unavailable if Xen doesn't find new
microcode to use.

Fixes: 648db37a155a ("x86/ucode: Distinguish "ucode already up to date"")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>