cifs: add function to get a tcon from cifs_sb

When we convert cifs to do multiple sessions per mount, we'll need more
than one tcon per superblock. At that point "cifs_sb->tcon" will make
no sense. Add a new accessor function that gets a tcon given a cifs_sb.
For now, it just returns cifs_sb->tcon. Later it'll do more.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
diff --git a/fs/cifs/cifs_dfs_ref.c b/fs/cifs/cifs_dfs_ref.c
index d6ced7a..f1e13ea 100644
--- a/fs/cifs/cifs_dfs_ref.c
+++ b/fs/cifs/cifs_dfs_ref.c
@@ -316,7 +316,7 @@
 	nd->path.dentry = dget(dentry);
 
 	cifs_sb = CIFS_SB(dentry->d_inode->i_sb);
-	ses = cifs_sb->tcon->ses;
+	ses = cifs_sb_tcon(cifs_sb)->ses;
 
 	if (!ses) {
 		rc = -EINVAL;