net: Fix 'Re: PACKET_TX_RING: packet size is too long'

Currently PACKET_TX_RING forces certain amount of every frame to remain
unused. This probably originates from an early version of the
PACKET_TX_RING patch that in fact used the extra space when the (since
removed) CONFIG_PACKET_MMAP_ZERO_COPY option was enabled. The current
code does not make any use of this extra space.

This patch removes the extra space reservation and lets userspace make
use of the full frame size.

Signed-off-by: Gabor Gombas <gombasg@sztaki.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>