clarify pgdat_to_phys() by wrapping pgdat_to_phys
with CONFIG_NUMA. (the same config as contig_page_data)

No functional change intended.

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/ (local)

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Miles Chen <redacted>
---
 mm/sparse.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index 7272f7a1449d..62c21ec28e33 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c

@@ -344,14 +344,15 @@ size_t mem_section_usage_size(void)
 	return sizeof(struct mem_section_usage) + usemap_size();
 }
 
-static inline phys_addr_t pgdat_to_phys(struct pglist_data *pgdat)
-{
-#ifndef CONFIG_NUMA
-	return __pa_symbol(pgdat);
-#else
-	return __pa(pgdat);
-#endif
-}
+#ifdef CONFIG_NUMA
+#define pgdat_to_phys(pgdat) __pa(pgdat)
+#else /* CONFIG_NUMA */
+/*
+ * When !CONFIG_NUMA, we only expect pgdat == &contig_page_data,
+ * and use __pa_symbol().
+ */
+#define pgdat_to_phys(pgdat) __pa_symbol(pgdat)
+#endif /* CONFIG_NUMA */