maple_tree: refine mab_calc_split function

Invert the conditional judgment of the mid_split, to focus the return
statement in the last statement, which is easier to understand and for
better readability.

Link: https://lkml.kernel.org/r/20221221060058.609003-8-vernon2gm@gmail.com
Signed-off-by: Vernon Yang <vernon2gm@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/maple_tree.c

@@ -1882,10 +1882,9 @@ static inline int mab_calc_split(struct ma_state *mas,
 
 	/* Avoid ending a node on a NULL entry */
 	split = mab_no_null_split(bn, split, slot_count);
-	if (!(*mid_split))
-		return split;
 
-	*mid_split = mab_no_null_split(bn, *mid_split, slot_count);
+	if (unlikely(*mid_split))
+		*mid_split = mab_no_null_split(bn, *mid_split, slot_count);
 
 	return split;
 }