Revert "datapath: Avoid null deref when GSO is for verifying header integrity only."

Revert "datapath: Avoid null deref when GSO is for verifying header integrity only."

This reverts commit 00c7faf3e5b7d4020e995a1429cf94313f197171.

In general, it should not be possible have a NULL return value from
skb_gso_segment() since we're not actually trying to verify the
header integrity.  No other callers with similar needs have NULL
checks.  The actual cause of the problem was LRO packets, which
OVS isn't equipped to handle.  The commit
33e031e99cc630baf1b0cb9256710dee7d9ab66d (datapath: Move LRO check
from transmit to receive.) solves that problem by fixing the LRO
check.  In order to avoid possibly masking any other problems, this
reverts the GSO check which should no longer be needed.

Signed-off-by: Jesse Gross <jesse@nicira.com>