vhost_user_fs: Add support for FOPEN_CACHE_DIR

Add support for FOPEN_CACHE_DIR, a flag that allows us to tell the
guest that it's safe to cache a directory, introduced in FUSE 7.28.

Signed-off-by: Sergio Lopez <slp@redhat.com>

vhost_user_fs/src/passthrough.rs

@@ -558,7 +558,13 @@ impl PassthroughFs {
                 OpenOptions::DIRECT_IO,
                 flags & (libc::O_DIRECTORY as u32) == 0,
             ),
-            CachePolicy::Always => opts |= OpenOptions::KEEP_CACHE,
+            CachePolicy::Always => {
+                if flags & (libc::O_DIRECTORY as u32) == 0 {
+                    opts |= OpenOptions::KEEP_CACHE;
+                } else {
+                    opts |= OpenOptions::CACHE_DIR;
+                }
+            }
             _ => {}
         };