staging: lustre: 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>
Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6714
Reviewed-on: http://review.whamcloud.com/15316
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6163
Reviewed-on: http://review.whamcloud.com/18819


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>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>