Revert "ipv6: make ip6_rt_gc_expire an atomic_t"

This reverts commit b4cfbeae which is
commit 9cb7c013420f98fa6fd12fc6a5dc055170c108db upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Idad46376e176f54a1d6781bdb432131bed7844f2
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>