llama : disable Direct IO by default (#19109)

* llama : disable Direct IO by default

* cont : override mmap if supported

include/llama.h

@@ -309,7 +309,7 @@ extern "C" {
         // Keep the booleans together to avoid misalignment during copy-by-value.
         bool vocab_only;      // only load the vocabulary, no weights
         bool use_mmap;        // use mmap if possible
-        bool use_direct_io;   // use direct io, takes precedence over use_mmap
+        bool use_direct_io;   // use direct io, takes precedence over use_mmap when supported
         bool use_mlock;       // force system to keep model in RAM
         bool check_tensors;   // validate model tensor data
         bool use_extra_bufts; // use extra buffer types (used for weight repacking)