mm: keep PGC_extra pages on a separate list

mm: keep PGC_extra pages on a separate list

This patch adds a new page_list_head into struct domain to hold PGC_extra
pages. This avoids them getting confused with 'normal' domheap pages where
the domain's page_list is walked.

A new dump loop is also added to dump_pageframe_info() to unconditionally
dump the 'extra page list'.

Signed-off-by: Paul Durrant <paul@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien@xen.org>