tracing/osnoise: Replace kmalloc + copy_from_user with memdup_user_nul

Replace kmalloc() followed by copy_from_user() with memdup_user_nul() to
simplify and improve osnoise_cpus_write(). Remove the manual
NUL-termination.

No functional changes intended.

Cc: Masami Hiramatsu <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Thorsten Blum <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

kernel/trace/trace_osnoise.c

···
 #include <linux/sched/clock.h>
 #include <uapi/linux/sched/types.h>
 #include <linux/sched.h>
+#include <linux/string.h>
 #include "trace.h"
 
 #ifdef CONFIG_X86_LOCAL_APIC
···
 	if (count < 1)
 		return 0;
 
-	buf = kmalloc(count + 1, GFP_KERNEL);
-	if (!buf)
-		return -ENOMEM;
-
-	if (copy_from_user(buf, ubuf, count))
-		return -EFAULT;
-	buf[count] = '\0';
+	buf = memdup_user_nul(ubuf, count);
+	if (IS_ERR(buf))
+		return PTR_ERR(buf);
 
 	if (!zalloc_cpumask_var(&osnoise_cpumask_new, GFP_KERNEL))
 		return -ENOMEM;