gsm: add osmo_mobile_identity_decode_from_l3_buf()

We have osmo_mobile_identity_decode_from_l3(), which takes a msgb as
argument, and decodes msg->l3h. Not all callers have their data in this
form. Offer a more flexible API for the same decoding.

For example, before the new function, osmo-hnbgw, which extracts a NAS
PDU from asn.1 packed data for CN pooling, would allocate a new msgb and
copy the NAS data just to pass a data pointer as argument.

Related: SYS#6412
Change-Id: I9bd99ccd01f0eedc091fe51687ff92ae1fdff60b
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 6795c57..0018e1c 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -391,6 +391,7 @@
 osmo_mobile_identity_to_str_c;
 osmo_mobile_identity_cmp;
 osmo_mobile_identity_decode;
+osmo_mobile_identity_decode_from_l3_buf;
 osmo_mobile_identity_decode_from_l3;
 osmo_mobile_identity_encoded_len;
 osmo_mobile_identity_encode_buf;