urcu: avoid false sharing for rcu_gp_ctr

@rcu_gp_ctr and @registry share the same cache line, it causes
false sharing and slowdown both of the read site and update site.

Fix: Use different cache line for them.

Although rcu_gp_futex is updated less than rcu_gp_ctr, but
they always be accessed at almost the same time, so we also move rcu_gp_futex
to the cacheline of rcu_gp_ctr to reduce the cacheline-usage or cache-missing
of read site.

test: (4X6=24 CPUs)

Before patch:

[root@localhost userspace-rcu]# ./tests/test_urcu_mb 20 1 20
SUMMARY ./tests/test_urcu_mb      testdur   20 nr_readers  20 rdur      0 wdur      0 nr_writers   1 wdelay      0 nr_reads   2100285330 nr_writes      3390219 nr_ops   2103675549
[root@localhost userspace-rcu]# ./tests/test_urcu_mb 20 1 20
SUMMARY ./tests/test_urcu_mb      testdur   20 nr_readers  20 rdur      0 wdur      0 nr_writers   1 wdelay      0 nr_reads   1619868562 nr_writes      3529478 nr_ops   1623398040
[root@localhost userspace-rcu]# ./tests/test_urcu_mb 20 1 20
SUMMARY ./tests/test_urcu_mb      testdur   20 nr_readers  20 rdur      0 wdur      0 nr_writers   1 wdelay      0 nr_reads   1949067038 nr_writes      3469334 nr_ops   1952536372

after patch:

[root@localhost userspace-rcu]# ./tests/test_urcu_mb 20 1 20
SUMMARY ./tests/test_urcu_mb      testdur   20 nr_readers  20 rdur      0 wdur      0 nr_writers   1 wdelay      0 nr_reads   3380191848 nr_writes      4903248 nr_ops   3385095096
[root@localhost userspace-rcu]# ./tests/test_urcu_mb 20 1 20
SUMMARY ./tests/test_urcu_mb      testdur   20 nr_readers  20 rdur      0 wdur      0 nr_writers   1 wdelay      0 nr_reads   3397637486 nr_writes      4129809 nr_ops   3401767295

Singed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>