mpls: In mpls_egress verify the packet length.

Reobert Shearman noticed that mpls_egress is failing to verify that
the bytes to be examined are in fact present in the packet before
mpls_egress reads those bytes.

As suggested by David Miller reduce this to a single pskb_may_pull
call so that we don't do unnecessary work in the fast path.

Reported-by: Robert Shearman <rshearma@brocade.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>