[NETFILTER] bridge: simplify nf_bridge_pad

Do some simple optimization on the nf_bridge_pad() function
and don't use magic constants. Eliminate a double call and
the #ifdef'd code for CONFIG_BRIDGE_NETFILTER.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>