lib/percpu_counter.c: __this_cpu_write() doesn't need to be protected by spinlock

__this_cpu_write doesn't need to be protected by spinlock, AS we are doing
per cpu write with preempt disabled.  And another reason to remove
__this_cpu_write outside of spinlock: __percpu_counter_sum is not an
accurate counter.

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>