staging: fsl-dpaa2/eth: Compute needed headroom per frame

For non-linear skbs we build scatter-gather frames and allocate
a new buffer for the S/G table in which we reserve the required
headroom, so the actual skb headroom size doesn't matter.

Rather than use a one-size-fits-all approach, decide when to
enforce headroom requirements on a frame by frame basis.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>