skb->tail in ibm_newemac should be skb_tail_pointer()

... since that sucker is not 32bit-only and on 64bit skb->tail is an
offset, not a pointer.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>