x86/mm: Introduce and use l?e_{get,from}_mfn()

x86/mm: Introduce and use l?e_{get,from}_mfn()

This avoids the explicit boxing/unboxing of mfn_t in relevant codepaths.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>