datapath: Fix mysterious GRE-over-IPSEC problems.

datapath: Fix mysterious GRE-over-IPSEC problems.

We've noticed that packets that go up to userspace and then back down to
the kernel and then enter an GRE tunnel that is then ESP encapsulated
by IPSEC end up with a bad ESP "next header" value: it ends up as zero
instead of 0x2f (IPPROTO_GRE).  Just putting packets from userspace into
a freshly allocated skb fixes the problem.

The underlying problem that this works around is still unknown.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Bug #4769.