[CIFS] cifs: fix oops on mount when CONFIG_CIFS_DFS_UPCALL is enabled

simple "mount -t cifs //xxx /mnt" oopsed on strlen of options
http://kerneloops.org/guilty.php?guilty=cifs_get_sb&version=2.6.25-release&start=16711 \
68&end=1703935&class=oops

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/cifsfs.c

@@ -97,9 +97,6 @@ cifs_read_super(struct super_block *sb, void *data,
 {
 	struct inode *inode;
 	struct cifs_sb_info *cifs_sb;
-#ifdef CONFIG_CIFS_DFS_UPCALL
-	int len;
-#endif
 	int rc = 0;
 
 	/* BB should we make this contingent on mount parm? */
@@ -117,15 +114,17 @@ cifs_read_super(struct super_block *sb, void *data,
 	 * complex operation (mount), and in case of fail
 	 * just exit instead of doing mount and attempting
 	 * undo it if this copy fails?*/
-	len = strlen(data);
-	cifs_sb->mountdata = kzalloc(len + 1, GFP_KERNEL);
-	if (cifs_sb->mountdata == NULL) {
-		kfree(sb->s_fs_info);
-		sb->s_fs_info = NULL;
-		return -ENOMEM;
+	if (data) {
+		int len = strlen(data);
+		cifs_sb->mountdata = kzalloc(len + 1, GFP_KERNEL);
+		if (cifs_sb->mountdata == NULL) {
+			kfree(sb->s_fs_info);
+			sb->s_fs_info = NULL;
+			return -ENOMEM;
+		}
+		strncpy(cifs_sb->mountdata, data, len + 1);
+		cifs_sb->mountdata[len] = '\0';
 	}
-	strncpy(cifs_sb->mountdata, data, len + 1);
-	cifs_sb->mountdata[len] = '\0';
 #endif
 
 	rc = cifs_mount(sb, cifs_sb, data, devname);