[NET]: Shut up warnings in net/core/flow.c

Not really a network problem, more a !SMP issue.

net/core/flow.c:295: warning: statement with no effect

flow.c:295:        smp_call_function(flow_cache_flush_per_cpu, &info, 1, 0);

Fix this by converting the macro to an inline function, which
also increases the typechecking for !SMP builds.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>