cifs: generate signkey for the channel that's reconnecting

BugLink: https://bugs.launchpad.net/bugs/2036450

Before my changes to how multichannel reconnects work, the
primary channel was always used to do a non-binding session
setup. With my changes, that is not the case anymore.
Missed this place where channel at index 0 was forcibly
updated with the signing key.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>
(cherry picked from commit 05ce044)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

Author: sprasad-microsoft

fs/cifs/smb2transport.c

@@ -425,7 +425,7 @@ generate_smb3signingkey(struct cifs_ses *ses,
 
 		/* safe to access primary channel, since it will never go away */
 		spin_lock(&ses->chan_lock);
-		memcpy(ses->chans[0].signkey, ses->smb3signingkey,
+		memcpy(ses->chans[chan_index].signkey, ses->smb3signingkey,
 		       SMB3_SIGN_KEY_SIZE);
 		spin_unlock(&ses->chan_lock);