smb3: missing lock when picking channel

Coverity spotted a place where we should have been holding the
channel lock when accessing the ses channel index.

Addresses-Coverity: 1582039 ("Data race condition (MISSING_LOCK)")
Cc: stable@vger.kernel.org
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/transport.c

@@ -1057,9 +1057,11 @@ struct TCP_Server_Info *cifs_pick_channel(struct cifs_ses *ses)
 		index = (uint)atomic_inc_return(&ses->chan_seq);
 		index %= ses->chan_count;
 	}
+
+	server = ses->chans[index].server;
 	spin_unlock(&ses->chan_lock);
 
-	return ses->chans[index].server;
+	return server;
 }
 
 int