udf: Use kvzalloc() in udf_sb_alloc_bitmap()

Use kvzalloc() in udf_sb_alloc_bitmap() instead of open-coding it.
Size computation wrapped in struct_size() macro to prevent potential
integer overflows.

Link: https://lore.kernel.org/r/20200827221652.64660-1-efremov@linux.com
Signed-off-by: Denis Efremov <efremov@linux.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/super.c

@@ -1006,18 +1006,10 @@ int udf_compute_nr_groups(struct super_block *sb, u32 partition)
 static struct udf_bitmap *udf_sb_alloc_bitmap(struct super_block *sb, u32 index)
 {
 	struct udf_bitmap *bitmap;
-	int nr_groups;
-	int size;
-
-	nr_groups = udf_compute_nr_groups(sb, index);
-	size = sizeof(struct udf_bitmap) +
-		(sizeof(struct buffer_head *) * nr_groups);
-
-	if (size <= PAGE_SIZE)
-		bitmap = kzalloc(size, GFP_KERNEL);
-	else
-		bitmap = vzalloc(size); /* TODO: get rid of vzalloc */
+	int nr_groups = udf_compute_nr_groups(sb, index);
 
+	bitmap = kvzalloc(struct_size(bitmap, s_block_bitmap, nr_groups),
+			  GFP_KERNEL);
 	if (!bitmap)
 		return NULL;