6lowpan: iphc: reset mac_header after decompress to fix panic

After decompression of 6lowpan socket data, an IPv6 header is inserted
before the existing socket payload.  After this, we reset the
network_header value of the skb to account for the difference in payload
size from prior to decompression + the addition of the IPv6 header.

However, we fail to reset the mac_header value.

Leaving the mac_header value untouched here, can cause a calculation
error in net/packet/af_packet.c packet_rcv() function when an
AF_PACKET socket is opened in SOCK_RAW mode for use on a 6lowpan
interface.

On line 2088, the data pointer is moved backward by the value returned
from skb_mac_header().  If skb->data is adjusted so that it is before
the skb->head pointer (which can happen when an old value of mac_header
is left in place) the kernel generates a panic in net/core/skbuff.c
line 1717.

This panic can be generated by BLE 6lowpan interfaces (such as bt0) and
802.15.4 interfaces (such as lowpan0) as they both use the sa...