container/init: op interface valid method

Check ops early and eliminate duplicate checks.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/initsymlink_test.go

@@ -3,6 +3,14 @@ package container
 import "testing"
 
 func TestSymlinkOp(t *testing.T) {
+	checkOpsValid(t, []opValidTestCase{
+		{"nil", (*SymlinkOp)(nil), false},
+		{"zero", new(SymlinkOp), false},
+		{"nil target", &SymlinkOp{LinkName: "/run/current-system"}, false},
+		{"zero linkname", &SymlinkOp{Target: MustAbs("/run/current-system")}, false},
+		{"valid", &SymlinkOp{Target: MustAbs("/run/current-system"), LinkName: "/run/current-system", Dereference: true}, true},
+	})
+
 	checkOpsBuilder(t, []opsBuilderTestCase{
 		{"current-system", new(Ops).Link(
 			MustAbs("/run/current-system"),