Btrfs: use the commit_root for reading free_space_inode crcs

Now that we are using regular file crcs for the free space cache,
we can deadlock if we try to read the free_space_inode while we are
updating the crc tree.

This commit fixes things by using the commit_root to read the crcs.  This is
safe because we the free space cache file would already be loaded if
that block group had been changed in the current transaction.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/btrfs_inode.h

@@ -187,4 +187,13 @@ static inline void btrfs_i_size_write(struct inode *inode, u64 size)
 	BTRFS_I(inode)->disk_i_size = size;
 }
 
+static inline bool btrfs_is_free_space_inode(struct btrfs_root *root,
+				       struct inode *inode)
+{
+	if (root == root->fs_info->tree_root ||
+	    BTRFS_I(inode)->location.objectid == BTRFS_FREE_INO_OBJECTID)
+		return true;
+	return false;
+}
+
 #endif