tracing: Replace strncpy with memcpy for fixed-length substring copy

checkpatch.pl reports the following warning:
WARNING: Prefer strscpy, strscpy_pad, or __nonstring over strncpy
(see: KSPP#90)

In synth_field_string_size(), replace strncpy() with memcpy() to copy 'len'
characters from 'start' to 'buf'. The code manually adds a NUL terminator
after the copy, making memcpy safe here.

Link: https://lore.kernel.org/20250325181232.38284-1-siddarthsgml@gmail.com
Signed-off-by: Siddarth G <siddarthsgml@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: Siddarth G

kernel/trace/trace_events_synth.c

@@ -207,7 +207,7 @@ static int synth_field_string_size(char *type)
 	if (len == 0)
 		return 0; /* variable-length string */
 
-	strncpy(buf, start, len);
+	memcpy(buf, start, len);
 	buf[len] = '\0';
 
 	err = kstrtouint(buf, 0, &size);