restore innodb_encrypt_tables validation function

that was apparently lost in 20c23048:

  commit 20c23048
  Author: Jan Lindström <jan.lindstrom@mariadb.com>
  Date:   Sun May 17 14:14:16 2015 +0300

      MDEV-8164: Server crashes in pfs_mutex_enter_func after fil_crypt_is_closing

This also reverts 8635c4b4:

  commit 8635c4b4
  Author: Jan Lindström <jan.lindstrom@mariadb.com>
  Date:   Thu May 21 11:02:03 2015 +0300

      Fix test failure.

mysql-test/suite/sys_vars/r/innodb_encrypt_tables_basic.result

@@ -19,17 +19,23 @@ where variable_name='innodb_encrypt_tables';
 VARIABLE_NAME	VARIABLE_VALUE
 INNODB_ENCRYPT_TABLES	OFF
 set global innodb_encrypt_tables=ON;
+ERROR 42000: Variable 'innodb_encrypt_tables' can't be set to the value of 'ON'
+show warnings;
+Level	Code	Message
+Warning	138	InnoDB: cannot enable encryption, encryption plugin is not available
+Error	1231	Variable 'innodb_encrypt_tables' can't be set to the value of 'ON'
 select @@global.innodb_encrypt_tables;
 @@global.innodb_encrypt_tables
-ON
+OFF
 set global innodb_encrypt_tables=OFF;
 select @@global.innodb_encrypt_tables;
 @@global.innodb_encrypt_tables
 OFF
 set global innodb_encrypt_tables=1;
+ERROR 42000: Variable 'innodb_encrypt_tables' can't be set to the value of '1'
 select @@global.innodb_encrypt_tables;
 @@global.innodb_encrypt_tables
-ON
+OFF
 set session innodb_encrypt_tables=1;
 ERROR HY000: Variable 'innodb_encrypt_tables' is a GLOBAL variable and should be set with SET GLOBAL
 set global innodb_encrypt_tables=1.1;

mysql-test/suite/sys_vars/t/innodb_encrypt_tables_basic.test

@@ -19,10 +19,13 @@ where variable_name='innodb_encrypt_tables';
 #
 # show that it's writable
 #
+--error ER_WRONG_VALUE_FOR_VAR
 set global innodb_encrypt_tables=ON;
+show warnings;
 select @@global.innodb_encrypt_tables;
 set global innodb_encrypt_tables=OFF;
 select @@global.innodb_encrypt_tables;
+--error ER_WRONG_VALUE_FOR_VAR
 set global innodb_encrypt_tables=1;
 select @@global.innodb_encrypt_tables;
 --error ER_GLOBAL_VARIABLE

storage/innobase/handler/ha_innodb.cc

@@ -19201,7 +19201,7 @@ static MYSQL_SYSVAR_ENUM(encrypt_tables, srv_encrypt_tables,
 			 PLUGIN_VAR_OPCMDARG,
 			 "Enable encryption for tables. "
 			 "Don't forget to enable --innodb-encrypt-log too",
-			 NULL,
+			 innodb_encrypt_tables_validate,
 			 innodb_encrypt_tables_update,
 			 0,
 			 &srv_encrypt_tables_typelib);

storage/xtradb/handler/ha_innodb.cc

@@ -20383,7 +20383,7 @@ static MYSQL_SYSVAR_ENUM(encrypt_tables, srv_encrypt_tables,
 			 PLUGIN_VAR_OPCMDARG,
 			 "Enable encryption for tables. "
 			 "Don't forget to enable --innodb-encrypt-log too",
-			 NULL,
+			 innodb_encrypt_tables_validate,
 			 innodb_encrypt_tables_update,
 			 0,
 			 &srv_encrypt_tables_typelib);