mm/sparse: clarify pgdat_to_phys

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2023396

This patch is a backport of the following upstream commit:
commit bdbda735508ca83341899a77f143e4d5c58007b3
Author: Miles Chen <miles.chen@mediatek.com>
Date:   Thu Sep 2 14:57:13 2021 -0700

    mm/sparse: clarify pgdat_to_phys

    Clarify pgdat_to_phys() by testing if
    pgdat == &contig_page_data when CONFIG_NUMA=n.

    We only expect contig_page_data in such case, so we
    use &contig_page_data directly instead of pgdat.

    No functional change intended when CONFIG_BUG_VM=n.

    Comment from Mark [1]:
    "
    ... and I reckon it'd be clearer and more robust to define
    pgdat_to_phys() in the same ifdefs as contig_page_data so
    that these, stay in-sync. e.g. have:

    | #ifdef CONFIG_NUMA
    | #define pgdat_to_phys(x)      virt_to_phys(x)
    | #else /* CONFIG_NUMA */
    |
    | extern struct pglist_data contig_page_data;
    | ...
    | #define pgdat_to_phys(x)      __pa_symbol(&contig_page_data)
    |
    | #endif /* CONIFIG_NUMA */
    "

    [1] https://lore.kernel.org/linux-arm-kernel/20210615131902.GB47121@C02TD0UTHF1T.local/

    Link: https://lkml.kernel.org/r/20210723123342.26406-1-miles.chen@mediatek.com
    Signed-off-by: Miles Chen <miles.chen@mediatek.com>
    Reviewed-by: David Hildenbrand <david@redhat.com>
    Acked-by: Mike Rapoport <rppt@linux.ibm.com>
    Cc: Mark Rutland <mark.rutland@arm.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Signed-off-by: Rafael Aquini <aquini@redhat.com>

mm/sparse.c

@@ -321,7 +321,8 @@ size_t mem_section_usage_size(void)
 static inline phys_addr_t pgdat_to_phys(struct pglist_data *pgdat)
 {
 #ifndef CONFIG_NUMA
-	return __pa_symbol(pgdat);
+	VM_BUG_ON(pgdat != &contig_page_data);
+	return __pa_symbol(&contig_page_data);
 #else
 	return __pa(pgdat);
 #endif