x86/mm: Combine {destroy,replace}_grant_{pte,va}_mapping()

x86/mm: Combine {destroy,replace}_grant_{pte,va}_mapping()

As with the create side of things, these are largely identical.  Most cases
are actually destroying the mapping rather than replacing it with a stolen
entry.

Reimplement their logic in replace_grant_pv_mapping() in a mostly common
way.

No (intended) change in behaviour from a guests point of view.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>