arm64: Add support for per-page flags

We need to store some extra information about a page, e.g. the state of
the MTE tags. Add a MD flags field to each page.

Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D55953

sys/arm64/arm64/pmap.c

@@ -1587,6 +1587,7 @@ pmap_page_init(vm_page_t m)
 
 	TAILQ_INIT(&m->md.pv_list);
 	m->md.pv_memattr = VM_MEMATTR_WRITE_BACK;
+	m->md.pv_flags = 0;
 }
 
 static void

sys/arm64/include/pmap.h

@@ -70,7 +70,8 @@ struct md_page {
 	TAILQ_HEAD(,pv_entry)	pv_list;
 	int			pv_gen;
 	vm_memattr_t		pv_memattr;
-	uint8_t			pv_reserve[3];
+	uint8_t			pv_flags;
+	uint8_t			pv_reserve[2];
 };
 
 enum pmap_stage {