switch the protection of percpu_counter list to spinlock

... making percpu_counter_destroy() non-blocking

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: zwliew <zhaoweiliew@gmail.com>

Conflicts:
	lib/percpu_counter.c

Change-Id: Ie6308a9efba369402ba7f38891ab5afbc6140b3f