LU-17000 coverity: llverfs: Add check for -n option

LU-17000 coverity: llverfs: Add check for -n option

CID: 397833 (Uninitialized scalar variable): llverfs.c
This patch also add missing "-n" opiton under man page

Test-Parameters: trivial testlist=sanity,conf-sanity
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I8c795edba0cfde80402db0f877caeb91663629a3
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52583
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>