switch the protection of percpu_counter list to spinlock

... making percpu_counter_destroy() non-blocking

Change-Id: Ida1bcd057aa43c024c1bc140880cac8d6f62fd82
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: zwliew <zhaoweiliew@gmail.com>