There is an unhandled corner case in the directory stream
handling. The stricter test catches it:

  go-fuse/fs$ go test
  --- FAIL: TestPosix (0.20s)
      --- FAIL: TestPosix/ReadDir (0.09s)
          test.go:342: got 19 entries, want 20
          test.go:342: got 40 entries, want 41
          test.go:342: got 61 entries, want 62
          test.go:342: got 82 entries, want 83
          test.go:342: got 103 entries, want 104
  FAIL
  exit status 1
  FAIL	github.com/hanwen/go-fuse/v2/fs	0.610s

Fix comes next.

Change-Id: I3e22904fce96bf2264ede47656ebd2936a7d13cd