bpf: let verifier to calculate and record max_pkt_offset

In check_packet_access, update max_pkt_offset after the offset has passed
__check_packet_access.

It should be safe to use u32 for max_pkt_offset as explained in code
comment.

Also, when there is tail call, the max_pkt_offset of the called program is
unknown, so conservatively set max_pkt_offset to MAX_PACKET_OFF for such
case.

Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>