gtp: parse extension headers before reading inner protocol

gtp: parse extension headers before reading inner protocol

GTPv1-U packets may carry a chain of extension headers before the inner
IP packet. The receive path already parses and skips these extension
headers, but it currently reads the inner protocol before doing so.

As a result, the first extension header byte is interpreted as the inner
IP version. Packets with extension headers are then dropped before PDP
lookup.

Parse the extension header chain before calling gtp_inner_proto(), so the
inner protocol is read from the actual inner IP header.

Fixes: c75fc0b9e5be ("gtp: identify tunnel via GTP device + GTP version + TEID + family")
Signed-off-by: Zhixing Chen <running910@gmail.com>
Link: https://patch.msgid.link/20260708042244.120898-1-running910@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>