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>