bridge: multicast: work around clang bug

bridge: multicast: work around clang bug

Clang-10 and clang-11 run into a corner case of the register
allocator on 32-bit ARM, leading to excessive stack usage from
register spilling:

net/bridge/br_multicast.c:2422:6: error: stack frame size of 1472 bytes in function 'br_multicast_get_stats' [-Werror,-Wframe-larger-than=]

Work around this by marking one of the internal functions as
noinline_for_stack.

Link: https://bugs.llvm.org/show_bug.cgi?id=45802#c9
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>