return GMM_CAUSE_IMSI_UNKNOWN if there is no auth data

If we have a subscriber entry that lacks auth data, we currently return
GMM_CAUSE_NET_FAIL, which in the MSC log looks like the HLR is not connected or
something grave. Instead, return GMM_CAUSE_IMSI_UNKNOWN.

This changes the OsmoMSC log in this way:
Before:

  DVLR <001e> VLR_Authenticate(901700000014701)[0x5555558dabb0]{VLR_SUB_AS_NEEDS_AUTH_WAIT_AI}: GSUP: rx Auth Info Error cause: 17: Network failure

After:

  DVLR <001e> VLR_Authenticate(901700000014701)[0x5555558dabb0]{VLR_SUB_AS_NEEDS_AUTH_WAIT_AI}: GSUP: rx Auth Info Error cause: 2: IMSI unknown in HLR

A better cause value would be something that says "IMSI known, but we have no
auth data", but since such cause value is not defined, the plain "IMSI unknown"
seems to be the best match.

Change-Id: I90df7b255317df1e5d968e7ce3b9d2c404b98db8
1 file changed