Btrfs: increase the global block reserve estimates

When doing IO with large amounts of data fragmentation, the global block
reserve calulations are too low.  This increases them to avoid
ENOSPC crashes.
Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -4108,7 +4108,7 @@ static u64 calc_global_metadata_size(struct btrfs_fs_info *fs_info)
 	num_bytes += div64_u64(data_used + meta_used, 50);
 
 	if (num_bytes * 3 > meta_used)
-		num_bytes = div64_u64(meta_used, 3);
+		num_bytes = div64_u64(meta_used, 3) * 2;
 
 	return ALIGN(num_bytes, fs_info->extent_root->leafsize << 10);
 }