[CIFS] redo existing session setup if needed in cifs_mount

When cifs_mount finds an existing SMB session that it can use for a new
mount, it does not check to see whether that session is in need of being
reconnected. An easy way to reproduce:

1) mount //server/share1
2) watch /proc/fs/cifs/DebugData for the share to go DISCONNECTED
3) mount //server/share2 with same creds as in step 1.

The second mount will fail because CIFSTCon returned -EAGAIN. If you do
an operation in share1 and then reattempt the mount it will work (since
the session is reestablished).

The following patch fixes this by having cifs_mount check the status
of the session when it picks an existing session and calling
cifs_setup_session on it again if it's in need of reconnection.

Thanks to Wojciech Pilorz for the initial bug report.

Signed-off-by: Jeff Layton <jlayton@tupile.poochiereds.net>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/connect.c

@@ -1964,7 +1964,15 @@ cifs_mount(struct super_block *sb, struct cifs_sb_info *cifs_sb,
 
 	if (existingCifsSes) {
 		pSesInfo = existingCifsSes;
-		cFYI(1, ("Existing smb sess found"));
+		cFYI(1, ("Existing smb sess found (status=%d)",
+			pSesInfo->status));
+		if (pSesInfo->status == CifsNeedReconnect) {
+			cFYI(1, ("Session needs reconnect"));
+			down(&pSesInfo->sesSem);
+			rc = cifs_setup_session(xid, pSesInfo,
+						cifs_sb->local_nls);
+			up(&pSesInfo->sesSem);
+		}
 	} else if (!rc) {
 		cFYI(1, ("Existing smb sess not found"));
 		pSesInfo = sesInfoAlloc();