btrfs: let ->s_fs_info point to fs_info, not root...

the latter can be obtained from the former (by looking as ->tree_root)
just as cheaply as we currently are doing the other way round.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/btrfs/export.c b/fs/btrfs/export.c
index 1b8dc33..5f77166 100644
--- a/fs/btrfs/export.c
+++ b/fs/btrfs/export.c
@@ -67,7 +67,7 @@
 				       u64 root_objectid, u32 generation,
 				       int check_generation)
 {
-	struct btrfs_fs_info *fs_info = btrfs_sb(sb)->fs_info;
+	struct btrfs_fs_info *fs_info = btrfs_sb(sb);
 	struct btrfs_root *root;
 	struct inode *inode;
 	struct btrfs_key key;