LU-6714 llog: fix the llog_cat_set_first_idx()

LU-6714 llog: fix the llog_cat_set_first_idx()

The bug was introduced in previous commit causing
wrong catalog index to be set.

Test-Parameters: alwaysuploadlogs envdefinitions=SLOW=yes mdtfilesystemtype=ldiskfs mdsfilesystemtype=ldiskfs ostfilesystemtype=ldiskfs clientdistro=el7 ossdistro=el6.6 mdsdistro=el6.6 mdtcount=1 testlist=performance-sanity
Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: Iaa0747c5f504109f3a265c7cdda91ebe7ba83b4f
Reviewed-on: http://review.whamcloud.com/15841
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>