LU-16518 libcfs: fix clang build errors

LU-16518 libcfs: fix clang build errors

Adjust a strncat and the preceding if statement
to account for the null terminator in the string.

Use (void) to designate two variables as unused
in a function to avoid doing a self-assign.

Also, use an explicit format to fix the format
security warning around alloc_workqueue.

Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I14a19ba83c063cd81c16723c31d0488c2b4f607e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50162
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>