mm: convert page type macros to enum

Changing PG_slab from a page flag to a page type in commit 46df8e73
("mm: free up PG_slab") in has the unintended consequence of removing the
PG_slab constant from kernel debuginfo.  The commit does add the value to
the vmcoreinfo note, which allows debuggers to find the value without
hardcoding it.  However it's most flexible to continue representing the
constant with an enum.  To that end, convert the page type fields into an
enum.  Debuggers will now be able to detect that PG_slab's type has
changed from enum pageflags to enum pagetype.

Link: https://lkml.kernel.org/r/20240607202954.1198180-1-stephen.s.brennan@oracle.com
Fixes: 46df8e73 ("mm: free up PG_slab")
Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hao Ge <gehao@kylinos.cn>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Omar Sandoval <osandov@osandov.com>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/page-flags.h

@@ -944,15 +944,18 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
  * mistaken for a page type value.
  */
 
-#define PAGE_TYPE_BASE	0xf0000000
-/* Reserve		0x0000007f to catch underflows of _mapcount */
-#define PAGE_MAPCOUNT_RESERVE	-128
-#define PG_buddy	0x00000080
-#define PG_offline	0x00000100
-#define PG_table	0x00000200
-#define PG_guard	0x00000400
-#define PG_hugetlb	0x00000800
-#define PG_slab		0x00001000
+enum pagetype {
+	PG_buddy	= 0x00000080,
+	PG_offline	= 0x00000100,
+	PG_table	= 0x00000200,
+	PG_guard	= 0x00000400,
+	PG_hugetlb	= 0x00000800,
+	PG_slab		= 0x00001000,
+
+	PAGE_TYPE_BASE	= 0xf0000000,
+	/* Reserve 0x0000007f to catch underflows of _mapcount */
+	PAGE_MAPCOUNT_RESERVE	= -128,
+};
 
 #define PageType(page, flag)						\
 	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)