affs: introduce VFS superblock object back-reference

Add an 'sb' VFS superblock back-reference to the 'struct affs_sb_info' data
structure - we will need to find the VFS superblock from a 'struct
affs_sb_info' object in the next patch, so this change is jut a preparation.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/affs/super.c b/fs/affs/super.c
index da7498d..0496cbb 100644
--- a/fs/affs/super.c
+++ b/fs/affs/super.c
@@ -299,6 +299,7 @@
 		return -ENOMEM;
 
 	sb->s_fs_info = sbi;
+	sbi->sb = sb;
 	mutex_init(&sbi->s_bmlock);
 	spin_lock_init(&sbi->symlink_lock);