mm/page_alloc.c: replace the definition of NR_MIGRATETYPE_BITS with PB_migratetype_bits

We already have the definition of PB_migratetype_bits and current
NR_MIGRATETYPE_BITS looks like a cyclic definition.

Just use PB_migratetype_bits is enough.
Signed-off-by: NWei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Link: http://lkml.kernel.org/r/20200623124201.8199-1-richard.weiyang@linux.alibaba.comSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -88,8 +88,7 @@ static inline bool is_migrate_movable(int mt)
 
 extern int page_group_by_mobility_disabled;
 
-#define NR_MIGRATETYPE_BITS (PB_migrate_end - PB_migrate + 1)
-#define MIGRATETYPE_MASK ((1UL << NR_MIGRATETYPE_BITS) - 1)
+#define MIGRATETYPE_MASK ((1UL << PB_migratetype_bits) - 1)
 
 #define get_pageblock_migratetype(page)					\
 	get_pfnblock_flags_mask(page, page_to_pfn(page),		\