btrfs: switch to kcalloc in btrfs_cmp_data_prepare

Kcalloc is functionally equivalent and does overflow checks.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -2925,8 +2925,8 @@ static int btrfs_cmp_data_prepare(struct inode *src, u64 loff,
 	 * of the array is bounded by len, which is in turn bounded by
 	 * BTRFS_MAX_DEDUPE_LEN.
 	 */
-	src_pgarr = kzalloc(num_pages * sizeof(struct page *), GFP_KERNEL);
-	dst_pgarr = kzalloc(num_pages * sizeof(struct page *), GFP_KERNEL);
+	src_pgarr = kcalloc(num_pages, sizeof(struct page *), GFP_KERNEL);
+	dst_pgarr = kcalloc(num_pages, sizeof(struct page *), GFP_KERNEL);
 	if (!src_pgarr || !dst_pgarr) {
 		kfree(src_pgarr);
 		kfree(dst_pgarr);