LU-12861 libcfs: Cleanup libcfs_debug_msg use of snprintf

LU-12861 libcfs: Cleanup libcfs_debug_msg use of snprintf

scnprintf returns the number of bytes written to the buffer.
snprintf returns the size of the buffer needed to satisfy
the request.

Prefer scnprintf when result is being used as the number
of bytes in a buffer.

Use snprintf when the result is used for sizing or resizing
a buffer.

Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I8c4b8f7dcc081f8b9dfffc35059011172be2e091
Reviewed-on: https://review.whamcloud.com/36901
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Ben Evans <beevans@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>