fsimpl: improve the "implements" comments

As noticed by @ayushr2, the "implements" comments are not
consistent, e.g.

// IterDirents implements kernfs.inodeDynamicLookup.
// Generate implements vfs.DynamicBytesSource.Generate.

This patch improves this by making the comments like this
consistently include the package name (when the interface
and struct are not in the same package) and method name.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>

pkg/sentry/fsimpl/tmpfs/filesystem.go

@@ -770,7 +770,7 @@ func (fs *filesystem) UnlinkAt(ctx context.Context, rp *vfs.ResolvingPath) error
 	return nil
 }
 
-// BoundEndpointAt implements FilesystemImpl.BoundEndpointAt.
+// BoundEndpointAt implements vfs.FilesystemImpl.BoundEndpointAt.
 func (fs *filesystem) BoundEndpointAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.BoundEndpointOptions) (transport.BoundEndpoint, error) {
 	fs.mu.RLock()
 	defer fs.mu.RUnlock()