netfilter: bridge: fix Kconfig unmet dependencies

Before f5efc69 ("netfilter: nf_tables: Add meta expression key for
bridge interface name"), the entire net/bridge/netfilter/ directory
depended on BRIDGE_NF_EBTABLES, ie. on ebtables. However, that
directory already contained the nf_tables bridge extension that
we should allow to compile separately. In f5efc69, we tried to
generalize this by using CONFIG_BRIDGE_NETFILTER which was not a good
idea since this option already existed and it is dedicated to enable
the Netfilter bridge IP/ARP filtering.

Let's try to fix this mess by:

1) making net/bridge/netfilter/ dependent on the toplevel
   CONFIG_NETFILTER option, just like we do with the net/netfilter and
   net/ipv{4,6}/netfilter/ directories.

2) Changing 'selects' to 'depends on' NETFILTER_XTABLES for
   BRIDGE_NF_EBTABLES. I believe this problem was already before
   f5efc69:

warning: (BRIDGE_NF_EBTABLES) selects NETFILTER_XTABLES which has
unmet direct dependencies (NET && INET && NETFILTER)

3) Fix ebtables/nf_tables bridge dependencies by making NF_TABLES_BRIDGE
   and BRIDGE_NF_EBTABLES dependent on BRIDGE and NETFILTER:

warning: (NF_TABLES_BRIDGE && BRIDGE_NF_EBTABLES) selects
BRIDGE_NETFILTER which has unmet direct dependencies (NET && BRIDGE &&
NETFILTER && INET && NETFILTER_ADVANCED)

net/built-in.o: In function `br_parse_ip_options':
br_netfilter.c:(.text+0x4a5ba): undefined reference to `ip_options_compile'
br_netfilter.c:(.text+0x4a5ed): undefined reference to `ip_options_rcv_srr'
net/built-in.o: In function `br_nf_pre_routing_finish':
br_netfilter.c:(.text+0x4a8a4): undefined reference to `ip_route_input_noref'
br_netfilter.c:(.text+0x4a987): undefined reference to `ip_route_output_flow'
make: *** [vmlinux] Error 1

Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>