mm/page_owner: use helper function zone_end_pfn() to get end_pfn

Commit 108bcc96

 ("mm: add & use zone_end_pfn() and zone_spans_pfn()")
introduced the helper zone_end_pfn() to calculate the zone end pfn.  But
pagetypeinfo_showmixedcount_print forgot to use it.  And the
initialization of local variable pfn is duplicated, remove one.

Link: https://lkml.kernel.org/r/20210123070538.5861-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>