check for skb headroom for packets out rmnet/bam interfaces

When packets destined out a rmnet/bam interface are sourced via the tun
interface, the skb has no headroom. This change checks for the no-headroom case
and uses skb_realloc_headroom

Without this patch, you get (panic+reboot):
[ 1522.386601] skb_under_panic: text:c0124e98 len:88 put:8 head:e077d800 data:e077d7f8 tail:0xe077d850 end:0xe077d880 dev:rmnet0
[ 1522.404028]I[0:          clatd: 7740] kernel BUG at /home/jenkins/android/workspace/android/jellybean/kernel/samsung/d2/net/core/skbuff.c:147!
...
[ 1524.974454]I[0:          clatd: 7740] Rebooting in 5 seconds..
[ 1529.990507]I[0:          clatd: 7740] Going down for restart now