percpu counter: clean up percpu_counter_sum_and_set()

percpu_counter_sum_and_set() and percpu_counter_sum() is the same except
the former updates the global counter after accounting.  Since we are
taking the fbc->lock to calculate the precise value of the counter in
percpu_counter_sum() anyway, it should simply set fbc->count too, as the
percpu_counter_sum_and_set() does.

This patch merges these two interfaces into one.
 
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>