tracing: Replace deprecated strncpy() with strscpy() for stack_trace_filter_buf

strncpy() is deprecated for NUL-terminated destination buffers and must
be replaced by strscpy().

See issue: https://github.com/KSPP/linux/issues/90

Link: https://lore.kernel.org/20250507133837.19640-1-devaanshk840@gmail.com


Signed-off-by: Devaansh Kumar <devaanshk840@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>