maple_tree: fix warning comparing pointer to 0

Avoid pointer type value compared with 0 to make code clear.

./tools/testing/radix-tree/maple.c:34142:15-16: WARNING comparing pointer to 0.

Link: https://lkml.kernel.org/r/[e-mail]
Reported-by: Abaci Robot <[e-mail]>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=7696
Signed-off-by: Jiapeng Chong <[e-mail]>
Cc: Liam R. Howlett <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/radix-tree/maple.c

···
 	mt_init_flags(mt, MT_FLAGS_ALLOC_RANGE);
 	check_erase2_testset(mt, set27, ARRAY_SIZE(set27));
 	rcu_barrier();
-	MT_BUG_ON(mt, 0 != mtree_load(mt, 140415537422336));
+	MT_BUG_ON(mt, NULL != mtree_load(mt, 140415537422336));
 	mt_set_non_kernel(0);
 	mt_validate(mt);
 	mtree_destroy(mt);
···
 	mt_init_flags(mt, MT_FLAGS_ALLOC_RANGE);
 	check_erase2_testset(mt, set37, ARRAY_SIZE(set37));
 	rcu_barrier();
-	MT_BUG_ON(mt, 0 != mtree_load(mt, 94637033459712));
+	MT_BUG_ON(mt, NULL != mtree_load(mt, 94637033459712));
 	mt_validate(mt);
 	mtree_destroy(mt);
 
···
 	mt_init_flags(mt, MT_FLAGS_ALLOC_RANGE);
 	check_erase2_testset(mt, set38, ARRAY_SIZE(set38));
 	rcu_barrier();
-	MT_BUG_ON(mt, 0 != mtree_load(mt, 94637033459712));
+	MT_BUG_ON(mt, NULL != mtree_load(mt, 94637033459712));
 	mt_validate(mt);
 	mtree_destroy(mt);