msm-5.4: CPE crash during SIP call

- Reset skb->len and skb->tail before skb_split.
- Use skb_nfct() helper instead of skb->_nfct.

Change-Id: I62f2ade5ebb4ecd197325e520223df4d6f6eff78
Signed-off-by: Dheeraj Kumar <quic_dhekum@quicinc.com>