Btrfs: Add block group pinned accounting back

This patch adds a helper function 'update_pinned_extents' to
extent-tree.c. The usage of the helper function is similar to
'update_block_group',  the last parameter of the function indicates
pin vs unpin.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 6c8533f..3e16cca 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -569,6 +569,7 @@
 			     fs_info->btree_inode->i_mapping, GFP_NOFS);
 	fs_info->do_barriers = 1;
 	fs_info->closing = 0;
+	fs_info->total_pinned = 0;
 
 	INIT_DELAYED_WORK(&fs_info->trans_work, btrfs_transaction_cleaner);
 	BTRFS_I(fs_info->btree_inode)->root = tree_root;