fuse: instead of ENOENT, return nil with op.Entry.Child == 0

The kernel treats a zero nodeid the same as ENOENT, but respects caching:

https://github.com/torvalds/linux/blob/2019fc96af228b412bdb2e8e0ad4b1fc12046a51/fs/fuse/dir.c#L353

This significantly reduces FUSE requests, e.g. during package builds:

before:
distri build -pkg=gtk+-2  438,42s user 160,10s system 569% cpu 1:45,12 total
distri build -pkg=gtk+-2  435,83s user 157,86s system 572% cpu 1:43,78 total

after:
distri build -pkg=gtk+-2  372,39s user 84,34s system 809% cpu 56,400 total
distri build -pkg=gtk+-2  373,22s user 85,02s system 812% cpu 56,366 total

related to #59

cmd/distri/internal/fuse/fuse.go

@@ -917,6 +917,7 @@ func (fs *fuseFS) LookUpInode(ctx context.Context, op *fuseops.LookUpInodeOp) er
 				}
 				return nil
 			}
+			// TODO: switch to returning nil once we invalidate the cache
 			return fuse.ENOENT
 		} else { // overlay directory
 			fs.mu.Lock()
@@ -927,7 +928,7 @@ func (fs *fuseFS) LookUpInode(ctx context.Context, op *fuseops.LookUpInodeOp) er
 			}
 			dirent, ok := dir.byName[op.Name]
 			if !ok {
-				return fuse.ENOENT
+				return nil // same as ENOENT when op.Entry.Child is 0
 			}
 			op.Entry.Child = dirent.inode
 			op.Entry.Attributes = fuseops.InodeAttributes{
@@ -975,7 +976,7 @@ func (fs *fuseFS) LookUpInode(ctx context.Context, op *fuseops.LookUpInodeOp) er
 
 	cie, ok := fis[op.Name]
 	if !ok {
-		return fuse.ENOENT
+		return nil // same as ENOENT when op.Entry.Child is 0
 	}
 	op.Entry.Child = cie.Child
 	op.Entry.Attributes = cie.Attributes