gprs_llc: Lookup lle based on the real TLLI

During the GPRS Attach procedure we might have a foreign tlli and
in the RX create a LLME on the fly for this tlli. The GMM GPRS
Attach handling code will then assign a new TLLI and keep the
foreign tlli as the llme->old_tlli.

When the GMM is sending the identity request the msgb_tlli will
point to the foreign tlli. The GPRS LLC code will then try to find
that foreign tlli but due the conversion this will not be found.
Instead a new ad-hoc LLE/LLME will be created on the fly for
each message (this means there are duplicate LLE/LLMEs in the
list).

Make the code more strict and remove the tlli_foreign2local change
from the look-up routine. This will make the GPRS LLC code find
the right LLE/LLME and the N(U) will be handled correctly.

This partially reverts:
  f0901f0067

Addresses:
  <0012> gprs_llc.c:773 LLC RX: unknown TLLI 0xadf11820, creating LLME on the fly
  ...
  <0012> gprs_llc.c:357 LLC TX: unknown TLLI 0xedf11820, creating LLME on the fly

Reproducable:
  Use pcu_emu (gprs attach) and observe with wireshark.

openbsc/src/gprs/gprs_llc.c

@@ -147,12 +147,10 @@ static inline uint32_t tlli_foreign2local(uint32_t tlli)
 }
 
 /* lookup LLC Entity based on DLCI (TLLI+SAPI tuple) */
-static struct gprs_llc_lle *lle_by_tlli_sapi(uint32_t tlli, uint8_t sapi)
+static struct gprs_llc_lle *lle_by_tlli_sapi(const uint32_t tlli, uint8_t sapi)
 {
 	struct gprs_llc_llme *llme;
 
-	tlli = tlli_foreign2local(tlli);
-
 	llist_for_each_entry(llme, &gprs_llc_llmes, list) {
 		if (llme->tlli == tlli || llme->old_tlli == tlli)
 			return &llme->lle[sapi];