gianfar: fix undo of reserve()

Fix undo of reserve() before RX recycle

gfar_new_skb reserve()s space in the SKB to align it.  If an error occurs,
and the skb needs to be returned to the RX recycle queue, the current code
attempts to reset head, but did not reset tail.  This patch remembers the
alignment amount, and reverses the reserve() when needed.

Signed-off-by: Ben Menchaca <ben@bigfootnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>