pmdagfs2: Make changes suggested by Nathan's patch for control.c

- the gfs2.control metrics seem to be defined as U32, so we should put
  'em into the pmAtomValue "ul" field (this only becomes visible on big
  endian boxes, benign otherwise).  gfs2_control_check_value comment is
  updated to match its existing return code behaviour.

- have also changed the pmAtomValue to ul for both CONTROL_WORSTGLOCK
  and CONTROL_FTRACE_GLOCK_THRESHOLD to match their U32 type also.

- tweaks an explicit tracing file location to use the table, and also
  get the compiler to work out the string size instead of hard-coding
  it in the strncmp call (40).