rmnet_core: Strengthen IPv6 fragment check in rmnet_frag_skip_exthdr()

Apparently, 4-6 XLAT scenarios can insert an empty fragment header into the
IPv6 header chain to indicate that fragmentation is supported in the host
IPv4 network. This header contains 0 for both the fragment_offset and MF
fields, so simply checking the value of frag_off passed to the
rmnet_frag_ipv6_skip_exthdr() function is not sufficient to properly catch
this rogue header. Instead, we need to implement a "less clever" version.

kernel oops at net/core/skbuff.c:4217!
Call trace:
 skb_segment+0xcf0/0xd2c
 __udp_gso_segment+0xa4/0x544
 udp6_ufo_fragment+0x2dc/0x344
 ipv6_gso_segment+0x170/0x350
 skb_mac_gso_segment+0xd4/0x1b0
 __skb_gso_segment+0xcc/0x12c
 udp_rcv_segment.76914+0x54/0x16c
 udpv6_queue_rcv_skb+0x78/0x148
 __udp6_lib_rcv+0x38c/0x4cc
 udpv6_rcv+0x20/0x30
 ip6_protocol_deliver_rcu+0x3c0/0x63c
 ip6_input+0x60/0x184
 ip6_rcv_finish+0x84/0x150
 ipv6_rcv+0x5c/0x14c
 __netif_receive_skb+0x80/0x184

Change-Id: Ica2779b1da17bc46d397b42283369f3750edbf82
Signed-off-by: Sean Tranchetti <quic_stranche@quicinc.com>
Signed-off-by: Sharath Chandra Vurukala <quic_sharathv@quicinc.com>