x86/mm: Use mfn_eq()/mfn_add() rather than opencoded variations

x86/mm: Use mfn_eq()/mfn_add() rather than opencoded variations

Use l1e_get_mfn() in place of l1e_get_pfn() when applicable, and fix up style
on affected lines.

For sh_remove_shadow_via_pointer(), map_domain_page() is guaranteed to succeed
so there is no need to ASSERT() its success.  This allows the pointer
arithmetic to folded into the previous expression, and for vaddr to be
properly typed as l1_pgentry_t, avoiding the cast in l1e_get_mfn().

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>