LU-6714 llog: fix wrong offset in llog_process_thread()

LU-6714 llog: fix wrong offset in llog_process_thread()

- llh_cat_idx may become bigger than llog bitmap size in
  llog_cat_set_first_idx() function
- it is wrong to use previous cur_offset as new buffer offset,
  new offset should be calculated from value returned by
  llog_next_block().
- optimize llog_skip_over() to find llog entry offset by index
  for llog with fixed-size records.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I0207ddd7e09161e25a2f5d56d69db4c993e650c3
Reviewed-on: http://review.whamcloud.com/15316
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>