6lowpan: Use skb_cow in IPHC decompression.

Currently there are potentially 2 skb_copy_expand calls in IPHC
decompression.  This patch replaces this with one call to
skb_cow which will check to see if there is enough headroom
first to ensure it's only done if necessary and will handle
alignment issues for cache.
As skb_cow uses pskb_expand_head we ensure the skb isn't shared from
bluetooth and ieee802.15.4 code that use the IPHC decompression.

Signed-off-by: Martin Townsend <martin.townsend@xsilon.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>