[NETNS][IPV6] ip6_fib - add net to gc timer parameter

      The fib tables are now relative to the network namespace. When the
garbage collector timer expires, we must have a network namespace
parameter in order to retrieve the tables. For now this is the
init_net, but we should be able to have a timer per namespace and use
the timer callback parameter to pass the network namespace from the
expired timer.

The timer callback, fib6_run_gc, is actually used to be called
synchronously by some functions and asynchronously when the timer
expires.

When the timer expires, the delay specified for fib6_run_gc parameter
is always zero. So, I changed fib6_run_gc to not be a timer callback
but a function called by the timer callback and I added a timer
callback where its work is just to retrieve from the data arg of the
timer the network namespace and call fib6_run_gc with zero expiring
time and the network namespace parameters. That makes the code cleaner
for the fib6_run_gc callers.

Signed-off-by: D