mm/sparse: fix "unused function 'pgdat_to_phys'" warning

W=1 build with clangs complains:

mm/sparse.c:347:27: warning: unused function 'pgdat_to_phys' [-Wunused-function]
static inline phys_addr_t pgdat_to_phys(struct pglist_data *pgdat)
                             ^
1 warning generated.

pgdat_to_phys() is only used by functions defined when
CONFIG_MEMORY_HOTREMOVE=y.

Move pgdat_to_phys() under #ifdef CONFIG_MEMORY_HOTREMOVE
to make clang happy.

Link: https://lkml.kernel.org/r/20230121101151.1703292-1-rppt@kernel.orgSigned-off-by: Mike Rapoport <rppt@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
  Link: https://lore.kernel.org/all/202301210155.1E5zABb5-lkp@intel.com
Cc: Miles Chen <miles.chen@mediatek.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/sparse.c

@@ -318,6 +318,7 @@ size_t mem_section_usage_size(void)
 	return sizeof(struct mem_section_usage) + usemap_size();
 }
 
+#ifdef CONFIG_MEMORY_HOTREMOVE
 static inline phys_addr_t pgdat_to_phys(struct pglist_data *pgdat)
 {
 #ifndef CONFIG_NUMA
@@ -328,7 +329,6 @@ static inline phys_addr_t pgdat_to_phys(struct pglist_data *pgdat)
 #endif
 }
 
-#ifdef CONFIG_MEMORY_HOTREMOVE
 static struct mem_section_usage * __init
 sparse_early_usemaps_alloc_pgdat_section(struct pglist_data *pgdat,
 					 unsigned long size)