quota: Convert quota statistics to generic percpu_counter

Generic per-cpu counter has some memory overhead but it is negligible for
modern systems and embedded systems compile without quota support.  And code
reuse is a good thing. This patch should fix complain from preemptive kernels
which was introduced by dde95888.

[Jan Kara: Fixed patch to work on 32-bit archs as well]

Reported-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jan Kara <jack@suse.cz>