Currently GTP packets are dropped if the next extension field is set to
non-zero value, but this are valid GTP packets.

TS 29.281 provides a longer header format, which is defined as struct
gtp1_header_long. Such long header format is used if any of the S, PN, E
flags is set.

This long header is 4 bytes longer than struct gtp1_header, plus
variable length (optional) extension headers. The next extension header
field is zero is no extension header is provided.

The extension header is composed of a length field which includes total
number of 4 byte words including the extension header itself (1 byte),
payload (variable length) and next type (1 byte). The extension header
size and its payload is aligned to 4 bytes.

A GTP packet might come with a chain extensions headers, which makes it
slightly cumbersome to parse because the extension next header field
comes at the end of the extension header, and there is a need to check
if this field becomes zero to stop the extension header parser.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>