bridge: Use consistent NF_DROP returns in nf_pre_routing

The nf_pre_routing functions in bridging have collected two
distinct ways of returning NF_DROP over the years, inline and
via goto.  There is no reason for preferring either one.

So this patch arbitrarily picks the inline variant and converts
the all the gotos.

Also removes a redundant comment.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>