LU-2726 procfs: Invalid assertion for counter_{add,sub}

lprocfs_stats::lc_sum_irq is only used for obd_memory for accurate
memory usage counting, other procfs stats counters do not need it,
even though they can be used in RCU callbacks.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I4c2dec50eecc0c63631f1915ba30a54a5da2fec1
Reviewed-on: http://review.whamcloud.com/5237
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John Hammond <johnlockwoodhammond@gmail.com>
Tested-by: Oleg Drokin <oleg.drokin@intel.com>