Move the allocation of SG skbs into the main path. This allows for
a little code sharing, and handling ENOMEM from within one place.

As side effect, L2 SG skbs now get the proper amount of additional
headroom (read: zero) instead of the hard-coded ETH_HLEN.
Signed-off-by: NJulian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>