Do not wipe keyslot area before in-place overwrite.

If LUKS2 keyslot area has to be overwritten (due to lack of free space),
do not wipe the affected area first. It will get overwritten anyway.
Originaly, in between the keyslot wipe and new key material write, pbkdf
calculation took place. The pbkdf calculation takes ~2 seconds by default
and it put the user in unnecesary risk of loosing the keysot data in case
of a crash.

lib/setup.c

@@ -4400,16 +4400,8 @@ int crypt_keyslot_change_by_passphrase(struct crypt_device *cd,
 			r = LUKS2_token_assignment_copy(cd, &cd->u.luks2.hdr, keyslot_old, keyslot_new, 0);
 			if (r < 0)
 				goto out;
-		} else {
+		} else
 			log_dbg(cd, "Key slot %d is going to be overwritten.", keyslot_old);
-			/* FIXME: improve return code so that we can detect area is damaged */
-			r = LUKS2_keyslot_wipe(cd, &cd->u.luks2.hdr, keyslot_old, 1);
-			if (r) {
-				/* (void)crypt_keyslot_destroy(cd, keyslot_old); */
-				r = -EINVAL;
-				goto out;
-			}
-		}
 
 		r = LUKS2_keyslot_store(cd,  &cd->u.luks2.hdr,
 					keyslot_new, new_passphrase,