[PPP]: handle misaligned accesses

From: "Philippe De Muyter" <phdm@macqel.be>

This patch avoids ppp-generated kernel crashes on machines where unaligned
accesses are forbidden (ie: m68000), by fixing ppp alignment setting for
reused skb's.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>