LU-10728 utils: fix str length in error string

LU-10728 utils: fix str length in error string

sizeof on pointers was used to get the length of the string. Use
instead string length from function inputs. Also remove useless uses
of snprintf and terminating null bytes.

Test-Parameters: @lnet
Signed-off-by: Cyril Bordage <cbordage@whamcloud.com>
Change-Id: I7053f39828ababd5782b360ef5c27c607ddb740d
Reviewed-on: https://review.whamcloud.com/40268
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>