kmsan: enter the runtime around kmsan_internal_memmove_metadata() call

kmsan_internal_memmove_metadata() transitively calls stack_depot_save()
(via kmsan_internal_chain_origin() and kmsan_save_stack_with_flags()),
which may allocate memory.  Guard it with kmsan_enter_runtime() and
kmsan_leave_runtime() to avoid recursion.

This bug was spotted by CONFIG_WARN_CAPABILITY_ANALYSIS=y

Link: https://lkml.kernel.org/r/20250507160012.3311104-4-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Acked-by: Marco Elver <elver@google.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/kmsan/hooks.c

@@ -275,8 +275,10 @@ void kmsan_copy_to_user(void __user *to, const void *from, size_t to_copy,
 		 * Don't check anything, just copy the shadow of the copied
 		 * bytes.
 		 */
+		kmsan_enter_runtime();
 		kmsan_internal_memmove_metadata((void *)to, (void *)from,
 						to_copy - left);
+		kmsan_leave_runtime();
 	}
 	user_access_restore(ua_flags);
 }