LU-13705 utils: fix llstat -n option

LU-13705 utils: fix llstat -n option

The '-n' option was not configured as a valid option for getopt
and would return an error if specified, instead of limiting the
number of stats outputs as it should.

Test-Parameters: trivial
Fixes: 3e0d994fbf4c ("LU-13705 utils: improve llstat/llobdstat usability")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ifacafce741854fe12b80ced28e95bc7cc9254035
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49289
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Feng Lei <flei@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>