mount: add -asyncDio flag for async direct I/O (#7922)

* mount: add -asyncDio flag for async direct I/O

This adds support for async direct I/O via the -asyncDio flag.

Async DIO enables the FUSE_CAP_ASYNC_DIO capability, allowing the kernel
to perform direct I/O operations asynchronously. This improves concurrency
for applications that use O_DIRECT flag.

Benefits:
- Better concurrency for direct I/O operations
- Improved performance for applications using O_DIRECT
- Reduced blocking on I/O operations

Use cases:
- Database workloads that use direct I/O
- Applications that bypass page cache intentionally
- High-performance I/O scenarios

Implementation inspired by JuiceFS which enables this capability
for improved I/O performance.

Usage:
  weed mount -filer=localhost:8888 -dir=/mnt/seaweedfs -asyncDio

* mount: add all remaining FUSE options (asyncDio, cacheSymlink, novncache)

This combines the remaining three FUSE mount options on top of the merged writebackCache PR:

1. asyncDio: Enable async direct I/O for better concurrency
2. cacheSymlink: Enable symlink caching to reduce metadata lookups
3. novncache: (macOS only) Disable vnode name caching to avoid stale data

All options use the function parameter 'option' instead of global 'mountOptions'.

weed/command/mount.go

@@ -50,6 +50,11 @@ type MountOptions struct {
 
 	// FUSE performance options
 	writebackCache *bool
+	asyncDio       *bool
+	cacheSymlink   *bool
+
+	// macOS-specific FUSE options
+	novncache *bool
 }
 
 var (
@@ -108,6 +113,11 @@ func init() {
 
 	// FUSE performance options
 	mountOptions.writebackCache = cmdMount.Flag.Bool("writebackCache", false, "enable FUSE writeback cache for improved write performance (at risk of data loss on crash)")
+	mountOptions.asyncDio = cmdMount.Flag.Bool("asyncDio", false, "enable async direct I/O for better concurrency")
+	mountOptions.cacheSymlink = cmdMount.Flag.Bool("cacheSymlink", false, "enable symlink caching to reduce metadata lookups")
+
+	// macOS-specific FUSE options
+	mountOptions.novncache = cmdMount.Flag.Bool("sys.novncache", false, "(macOS only) disable vnode name caching to avoid stale data")
 }
 
 var cmdMount = &Command{