afsmonitor: Fix multiple NUM_FS_STAT_ENTRIES overflows

If an array is n elements long, accessing element array[n] is an
overflow. Fix various places where we apply loop bounds incorrectly
using the NUM_FS_STAT_ENTRIES constant.

Caught by coverity (#985570, #985571, #985572)

Change-Id: I8a28f06059771f91415ebc989714929cfd09f296
Reviewed-on: http://gerrit.openafs.org/9315
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>