simplify bitwise checking (#30722)

Some of these checks before bitwise operations are redundant and compilers
do not always recognize them, so let's simplify the code to make the intentions
clearer.

src/shared/mount-util.c

@@ -821,8 +821,8 @@ int mount_option_mangle(
 
                         if (!(ent->mask & MNT_INVERT))
                                 mount_flags |= ent->id;
-                        else if (mount_flags & ent->id)
-                                mount_flags ^= ent->id;
+                        else
+                                mount_flags &= ~ent->id;
 
                         break;
                 }