implement NoOpenSupport and NoOpendirSupport

See also:
https://github.com/torvalds/linux/commit/7678ac50615d9c7a491d9861e020e4f5f71b594c
https://github.com/torvalds/linux/commit/d7afaec0b564f0609e116f562983b8e72fc3e9c9
https://github.com/torvalds/linux/commit/d9a9ea94f748f47b1d75c6c5e33edcf74476c445

connection.go

@@ -149,6 +149,8 @@ func (c *Connection) Init() error {
 	}
 
 	cacheSymlinks := initOp.Flags&fusekernel.InitCacheSymlinks > 0
+	noOpenSupport := initOp.Flags&fusekernel.InitNoOpenSupport > 0
+	noOpendirSupport := initOp.Flags&fusekernel.InitNoOpendirSupport > 0
 
 	// Respond to the init op.
 	initOp.Library = c.protocol
@@ -171,6 +173,18 @@ func (c *Connection) Init() error {
 		initOp.Flags |= fusekernel.InitCacheSymlinks
 	}
 
+	// Tell the kernel to treat returning -ENOSYS on OpenFile as not needing
+	// OpenFile calls at all (Linux >= 3.16):
+	if c.cfg.EnableNoOpenSupport && noOpenSupport {
+		initOp.Flags |= fusekernel.InitNoOpenSupport
+	}
+
+	// Tell the kernel to treat returning -ENOSYS on OpenDir as not needing
+	// OpenDir calls at all (Linux >= 5.1):
+	if c.cfg.EnableNoOpendirSupport && noOpendirSupport {
+		initOp.Flags |= fusekernel.InitNoOpendirSupport
+	}
+
 	c.Reply(ctx, nil)
 	return nil
 }

internal/fusekernel/fuse_kernel.go

@@ -267,6 +267,7 @@ const (
 	InitWritebackCache   InitFlags = 1 << 16
 	InitNoOpenSupport    InitFlags = 1 << 17
 	InitCacheSymlinks    InitFlags = 1 << 23
+	InitNoOpendirSupport InitFlags = 1 << 24
 
 	InitCaseSensitive InitFlags = 1 << 29 // OS X only
 	InitVolRename     InitFlags = 1 << 30 // OS X only
@@ -298,6 +299,7 @@ var initFlagNames = []flagName{
 	{uint32(InitWritebackCache), "InitWritebackCache"},
 	{uint32(InitNoOpenSupport), "InitNoOpenSupport"},
 	{uint32(InitCacheSymlinks), "InitCacheSymlinks"},
+	{uint32(InitNoOpendirSupport), "InitNoOpendirSupport"},
 
 	{uint32(InitCaseSensitive), "InitCaseSensitive"},
 	{uint32(InitVolRename), "InitVolRename"},

mount_config.go

@@ -139,6 +139,18 @@ type MountConfig struct {
 	// target.
 	EnableSymlinkCaching bool
 
+	// Linux only.
+	//
+	// Tell the kernel to treat returning -ENOSYS on OpenFile as not needing
+	// OpenFile calls at all (Linux >= 3.16):
+	EnableNoOpenSupport bool
+
+	// Linux only.
+	//
+	// Tell the kernel to treat returning -ENOSYS on OpenDir as not needing
+	// OpenDir calls at all (Linux >= 5.1):
+	EnableNoOpendirSupport bool
+
 	// OS X only.
 	//
 	// The name of the mounted volume, as displayed in the Finder. If empty, a