cap: drop existent item before validating

Signed-off-by: Ma Shimiao <mashimiao.fnst@cn.fujitsu.com>

Author: Ma Shimiao

generate/generate.go

@@ -1050,9 +1050,6 @@ func (g *Generator) AddProcessCapability(c string) error {
 // DropProcessCapability drops a process capability from g.spec.Process.Capabilities.
 func (g *Generator) DropProcessCapability(c string) error {
 	cp := strings.ToUpper(c)
-	if err := validate.CapValid(cp, false); err != nil {
-		return err
-	}
 
 	g.initSpecProcessCapabilities()
 
@@ -1092,6 +1089,10 @@ func (g *Generator) DropProcessCapability(c string) error {
 		}
 	}
 
+	if err := validate.CapValid(cp, false); err != nil {
+		return err
+	}
+
 	return nil
 }