rust: helpers: Add memchr wrapper for string operations

Add a dedicated string helper file with a memchr wrapper that uses the
kernel's instrumented memchr() function to ensure KASAN and FORTIFY_SOURCE
protections are preserved for Rust code.

Reported-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Link: https://lore.kernel.org/rust-for-linux/CANiq72mXAZc0sNM7ShX8VDVs_7zJddawP-e=wt+ERr1YUCcWUw@mail.gmail.com/
Signed-off-by: Jan Polensky <japo@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

rust/helpers/helpers.c

@@ -88,6 +88,7 @@
 #include "signal.c"
 #include "slab.c"
 #include "spinlock.c"
+#include "string.c"
 #include "sync.c"
 #include "task.c"
 #include "time.c"

rust/helpers/string.c

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/string.h>
+
+__rust_helper void *rust_helper_memchr(const void *s, int c, size_t n)
+{
+	return memchr(s, c, n);
+}