bnxt_en: Fix and improve .ndo_features_check().

Jakub Kicinski pointed out that we need to handle ipv6 extension headers
and to explicitly check for supported tunnel types in
.ndo_features_check().

For ipv6 extension headers, the hardware supports up to 2 ext. headers
and each must be <= 64 bytes.  For tunneled packets, the supported
packets are UDP with supported VXLAN and Geneve ports, GRE, and IPIP.

v3: More improvements based on Alexander Duyck's valuable feedback -
    Remove the jump lable in bnxt_features_check() and restructure it
    so that the TCP/UDP is check is consolidated in bnxt_exthdr_check().

v2: Add missing step to check inner ipv6 header for UDP and GRE tunnels.
    Check TCP/UDP next header after skipping ipv6 ext headers for
    non-tunneled packets and for inner ipv6.
    (Both feedback from Alexander Duyck)

Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Fixes: 1698d600 ("bnxt_en: Implement .ndo_features_check().")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>