tsacct: replace strncpy() with strscpy()

Replace the deprecated[1] use of strncpy() in bacct_add_tsk().  Since this
is UAPI, include trailing padding in the copy.

Link: https://github.com/KSPP/linux/issues/90 [1]
Link: https://lkml.kernel.org/r/20240711171308.work.995-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>
Cc: "Dr. Thomas Orgis" <thomas.orgis@uni-hamburg.de>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Ismael Luceno <ismael@iodev.co.uk>
Cc: Peng Liu <liupeng256@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>