smb3: fix typo in mount options displayed in /proc/mounts

Missing the final 's' in "max_channels" mount option when displayed in
/proc/mounts (or by mount command)

CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Shyam Prasad N <nspmangalore@gmail.com>

fs/cifs/cifsfs.c

@@ -623,7 +623,7 @@ cifs_show_options(struct seq_file *s, struct dentry *root)
 	seq_printf(s, ",actimeo=%lu", cifs_sb->actimeo / HZ);
 
 	if (tcon->ses->chan_max > 1)
-		seq_printf(s, ",multichannel,max_channel=%zu",
+		seq_printf(s, ",multichannel,max_channels=%zu",
 			   tcon->ses->chan_max);
 
 	return 0;