Revert "ipv6: make ip6_rt_gc_expire an atomic_t"

This reverts commit 49516e6e which is
commit 9cb7c013 upstream.

It breaks the Android KABI and is not needed in Android devices, so just
revert it.

Bug: 161946584
Fixes: 49516e6e ("ipv6: make ip6_rt_gc_expire an atomic_t")
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Ibab89760234525cc5622cd60e61599784fd0f80a