bridge: netfilter: Move sysctl-specific error code inside #ifdef

If CONFIG_SYSCTL=n:

    net/bridge/br_netfilter.c: In function ‘br_netfilter_init’:
    net/bridge/br_netfilter.c:996: warning: label ‘err1’ defined but not used

Move the label and the code after it inside the existing #ifdef to get
rid of the warning.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>