Code generation when Clang 17 changed, and it generates some code that
is problematic for the verifier, even though it should be correct
logically.

More specifically, when nodeport_lb6 calls lb6_extract_tuple, which
calls ipv6_hdrlen_offset, the latter might return an error, in which
case l4_off in nodeport_lb6 remains non-initialized. If that happens,
nodeport_lb6 returns early. The error code when it can possibly continue
is DROP_UNSUPP_SERVICE_PROTO, but it's only returned by
lb6_extract_tuple after l4_off is initialized, so we are safe here.
Moreover, is_svc_proto is set to false in this case, which blocks
further l4_off access.

However, Clang 17 generates code which jumps to a common chunk after
lb6_extract_tuple returns, and this chunk starts from spilling R2 to the
stack: `*(u32 *)(r10 -176) = r2`. R2 stores l4_off, but only after it
has been assigned, otherwise it's not initialized, and this is what the
verifier complains for, even though it's not used afterwards on errors.

Work around it by always assiging l4_off in lb6_extract_tuple.

Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>