n_gsm.c: Implement 3GPP27.010 DLC start-up procedure in MUX

In 3GPP27.010 5.8.1, it defined:
The TE multiplexer initiates the establishment of the multiplexer control channel by sending a SABM frame on DLCI 0 using the procedures of clause 5.4.1.
Once the multiplexer channel is established other DLCs may be established using the procedures of clause 5.4.1.
This patch implement 5.8.1 in MUX level, it make sure DLC0 is the first channel to be setup.

[or for those not familiar with the specification: it was possible to try
 and open a data connection while the control channel was not yet fully
 open, which is a spec violation and confuses some modems]
Signed-off-by: Nxiaojin <jin.xiao@intel.com>
Tested-by: NYin, Fengwei <fengwei.yin@intel.com>
[tweaked the order we check things and error code]
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Cc: The Horsebox <stable@vger.kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -2889,6 +2889,10 @@ static int gsmtty_install(struct tty_driver *driver, struct tty_struct *tty)
 	gsm = gsm_mux[mux];
 	if (gsm->dead)
 		return -EL2HLT;
+	/* If DLCI 0 is not yet fully open return an error. This is ok from a locking
+	   perspective as we don't have to worry about this if DLCI0 is lost */
+	if (gsm->dlci[0] && gsm->dlci[0]->state != DLCI_OPEN) 
+		return -EL2NSYNC;
 	dlci = gsm->dlci[line];
 	if (dlci == NULL) {
 		alloc = true;