staging: lustre: llog: prevent out-of-bound index

llog_process_thread() can be called from llog_cat_process_cb with an
index already out of bound, leading to the following crash:

LustreError: 3773:0:(llog.c:310:llog_process_thread())
  ASSERTION(index <= last_index + 1 ) failed:
LustreError: 3773:0:(llog.c:310:llog_process_thread()) LBUG

 #0 [ffff8801144bf900] machine_kexec at ffffffff81038f3b
 #1 [ffff8801144bf960] crash_kexec at ffffffff810c5d82
 #2 [ffff8801144bfa30] panic at ffffffff8152798a
 #3 [ffff8801144bfab0] lbug_with_loc at ffffffffa02f8eeb [libcfs]
 #4 [ffff8801144bfad0] llog_process_thread at ffffffffa0413fff [obdclass]
 #5 [ffff8801144bfb80] llog_process_or_fork at ffffffffa041585f [obdclass]
 #6 [ffff8801144bfbd0] llog_cat_process_cb at ffffffffa0418612 [obdclass]
 #7 [ffff8801144bfc30] llog_process_thread at ffffffffa0413c22 [obdclass]
 #8 [ffff8801144bfce0] llog_process_or_fork at ffffffffa041585f [obdclass]
 #9 [ffff8801144bfd30] llog_cat_process_or_fork at ffffffffa0416b9d [obdclass]

If index is too big, simply return success.

Signed-off-by: frank zago <fzago@cray.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5635
Reviewed-on: http://review.whamcloud.com/12161
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: John L. Hammond <john.hammond@intel.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>