xtensa: use generic strncpy_from_user with KASAN

This enables KASAN check of the destination buffer.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/kernel/xtensa_ksyms.c

@@ -44,7 +44,9 @@ EXPORT_SYMBOL(memmove);
 EXPORT_SYMBOL(__memset);
 EXPORT_SYMBOL(__memcpy);
 EXPORT_SYMBOL(__memmove);
+#ifndef CONFIG_GENERIC_STRNCPY_FROM_USER
 EXPORT_SYMBOL(__strncpy_user);
+#endif
 EXPORT_SYMBOL(clear_page);
 EXPORT_SYMBOL(copy_page);