tests/reverse: add TestXattrGetMountpoint

Fails right now:

gocryptfs/tests/reverse$ go test -run TestXattrGetMountpoint
--- FAIL: TestXattrGetMountpoint (0.00s)
    xattr_test.go:78: LGet: xattr.get /var/tmp/gocryptfs-test-parent-1026/3269496664/x.3903415542.mnt user.foo453465324: invalid argument
    xattr_test.go:82: LList: err=<nil>
FAIL
testcases[0] = struct { plaintextnames bool; deterministic_names bool }{plaintextnames:false, deterministic_names:false} failed
exit status 1
FAIL	github.com/rfjakob/gocryptfs/v2/tests/reverse	0.091s

1 files changed, 16 insertions, 0 deletions

diff --git a/tests/reverse/xattr_test.go b/tests/reverse/xattr_test.go
index 406d055..b6a7b34 100644
--- a/tests/reverse/xattr_test.go
+++ b/tests/reverse/xattr_test.go
@@ -9,6 +9,7 @@ import (
 	"testing"
 
 	"github.com/pkg/xattr"
+	"golang.org/x/sys/unix"
 )
 
 func xattrSupported(path string) bool {
@@ -65,3 +66,18 @@ func TestXattrList(t *testing.T) {
 		}
 	}
 }
+
+// Shouldn't get EINVAL when querying the mountpoint.
+func TestXattrGetMountpoint(t *testing.T) {
+	_, err := xattr.LGet(dirB, "user.foo453465324")
+	if err == nil {
+		return
+	}
+	e2 := err.(*xattr.Error)
+	if e2.Unwrap() == unix.EINVAL {
+		t.Errorf("LGet: %v", err)
+	}
+	// Let's see what LList says
+	_, err = xattr.LList(dirB)
+	t.Logf("LList: err=%v", err)
+}