Btrfs: Add 1MB to the min_free in alloc_chunk

This properly reflects the first 1MB we skip at the start of the device
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/volumes.c

@@ -746,6 +746,9 @@ int btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 	else
 		min_free = calc_size;
 
+	/* we add 1MB because we never use the first 1MB of the device */
+	min_free += 1024 * 1024;
+
 	/* build a private list of devices we will allocate from */
 	while(index < num_stripes) {
 		device = list_entry(cur, struct btrfs_device, dev_list);