x86/mm: carve out replace_grant_pv_mapping

x86/mm: carve out replace_grant_pv_mapping

And at once make it an inline function. Add declarations of
replace_grant_{p2m,pv}_mapping to respective header files.

The code movement will be done later.

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