cifs: fix unitialized variable poential problem with network I/O cache lock patch

static analysis with Coverity detected an issue with the following
commit:

 Author: Paulo Alcantara (SUSE) <pc@cjr.nz>
 Date:   Wed Dec 4 17:38:03 2019 -0300

    cifs: Avoid doing network I/O while holding cache lock

Addresses-Coverity: ("Uninitialized pointer read")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/dfs_cache.c

@@ -1349,7 +1349,7 @@ static struct cifs_ses *find_root_ses(struct vol_info *vi,
 	char *mdata = NULL, *devname = NULL;
 	struct TCP_Server_Info *server;
 	struct cifs_ses *ses;
-	struct smb_vol vol;
+	struct smb_vol vol = {NULL};
 
 	rpath = get_dfs_root(path);
 	if (IS_ERR(rpath))