gnttab: consolidate pin-to-status syncing

gnttab: consolidate pin-to-status syncing

Forever since the fix for XSA-230 the 2nd of the comments ahead of
fixup_status_for_copy_pin() has been stale - there's nothing specific to
transitive grants there anymore.

Move the function up, drop the "copy" part from its name again, add a
"readonly" parameter, and use it also on other paths having decremented
one (or not having got to increment any) of the pin counts.

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