staging: zram: drop zram_stat_dec/inc functions

It seems like an overkill to have adding and subtracting
1 functions from the 32bit counters. Just do it directly.

Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>