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: KSPP#90 [1]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>
Cc: "Dr. Thomas Orgis" <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Ismael Luceno <[email protected]>
Cc: Peng Liu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

kernel/tsacct.c

@@ -76,7 +76,7 @@ void bacct_add_tsk(struct user_namespace *user_ns,
 	stats->ac_minflt = tsk->min_flt;
 	stats->ac_majflt = tsk->maj_flt;
 
-	strncpy(stats->ac_comm, tsk->comm, sizeof(stats->ac_comm));
+	strscpy_pad(stats->ac_comm, tsk->comm);
 }