netfilter: bridge: avoid unused label warning

With the ARM mini2440_defconfig, the bridge netfilter code gets
built with both CONFIG_NF_DEFRAG_IPV4 and CONFIG_NF_DEFRAG_IPV6
disabled, which leads to a harmless gcc warning:

net/bridge/br_netfilter_hooks.c: In function 'br_nf_dev_queue_xmit':
net/bridge/br_netfilter_hooks.c:792:2: warning: label 'drop' defined but not used [-Wunused-label]

This gets rid of the warning by cleaning up the code to avoid
the respective #ifdefs causing this problem, and replacing them
with if(IS_ENABLED()) checks. I have verified that the resulting
object code is unchanged, and an additional advantage is that
we now get compile coverage of the unused functions in more
configurations.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: dd302b59bde0 ("netfilter: bridge: don't leak skb in error paths")
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>