Move hostfs mount to Kernel struct.

This is needed to set up host fds passed through a Unix socket. Note that
the host package depends on kernel, so we cannot set up the hostfs mount
directly in Kernel.Init as we do for sockfs and pipefs.

Also, adjust sockfs to make its setup look more like hostfs's and pipefs's.

PiperOrigin-RevId: 308274053

pkg/sentry/fsimpl/host/host.go

@@ -42,7 +42,7 @@ type filesystemType struct{}
 
 // GetFilesystem implements FilesystemType.GetFilesystem.
 func (filesystemType) GetFilesystem(context.Context, *vfs.VirtualFilesystem, *auth.Credentials, string, vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
-	panic("cannot instaniate a host filesystem")
+	panic("host.filesystemType.GetFilesystem should never be called")
 }
 
 // Name implements FilesystemType.Name.
@@ -55,14 +55,14 @@ type filesystem struct {
 	kernfs.Filesystem
 }
 
-// NewMount returns a new disconnected mount in vfsObj that may be passed to ImportFD.
-func NewMount(vfsObj *vfs.VirtualFilesystem) (*vfs.Mount, error) {
+// NewFilesystem sets up and returns a new hostfs filesystem.
+//
+// Note that there should only ever be one instance of host.filesystem,
+// a global mount for host fds.
+func NewFilesystem(vfsObj *vfs.VirtualFilesystem) *vfs.Filesystem {
 	fs := &filesystem{}
-	fs.Init(vfsObj, &filesystemType{})
-	vfsfs := fs.VFSFilesystem()
-	// NewDisconnectedMount will take an additional reference on vfsfs.
-	defer vfsfs.DecRef()
-	return vfsObj.NewDisconnectedMount(vfsfs, nil, &vfs.MountOptions{})
+	fs.Init(vfsObj, filesystemType{})
+	return fs.VFSFilesystem()
 }
 
 // ImportFD sets up and returns a vfs.FileDescription from a donated fd.