x86/mm: clean up SHARED_M2P{,_ENTRY} uses

x86/mm: clean up SHARED_M2P{,_ENTRY} uses

Stop open-coding SHARED_M2P() and drop a pointless use of it from
paging_mfn_is_dirty() (!VALID_M2P() is a superset of SHARED_M2P()) and
another one from free_page_type() (prior assertions render this
redundant).

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