mempool.c: clean up type-casting

clean up type-casting twice.  "size_t" is typedef as "unsigned long" in
64-bit system, and "unsigned int" in 32-bit system, and the intermediate
cast to 'long' is pointless.
Signed-off-by: Figo.zhang <figo1802@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempool.c

@@ -303,14 +303,14 @@ EXPORT_SYMBOL(mempool_free_slab);
  */
 void *mempool_kmalloc(gfp_t gfp_mask, void *pool_data)
 {
-	size_t size = (size_t)(long)pool_data;
+	size_t size = (size_t)pool_data;
 	return kmalloc(size, gfp_mask);
 }
 EXPORT_SYMBOL(mempool_kmalloc);
 
 void *mempool_kzalloc(gfp_t gfp_mask, void *pool_data)
 {
-	size_t size = (size_t) pool_data;
+	size_t size = (size_t)pool_data;
 	return kzalloc(size, gfp_mask);
 }
 EXPORT_SYMBOL(mempool_kzalloc);