datapath: Update inner offsets when expanding headroom.

datapath: Update inner offsets when expanding headroom.

skb protocol offsets are relative to the beginning of the
buffer and therefore must be updated if the buffer size is
expanded. Kernel functions do this automatically for existing
fields but obviously not for anything that we backport. This
introduces a wrapper for pskb_expand_head() to update the
inner protocol fields that we have backported.

Without this, a kernel crash can be triggered with tunnel
packets that do not have enough headroom and need to be
segmented. pskb_expand_head() is called in directly through
skb_cow_head() at the beginning of each of the tunnel transmit
routines.

Reported-by: Yinpeijun <yinpeijun@huawei.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>