MDEV-11782 WIP: Support upgrade from MariaDB 10.1.

recv_log_format_0_recover(): Invoke log_decrypt_after_read() after
reading the old-format redo log buffer.

With this change, we will upgrade to an encrypted redo log that
is misleadingly carrying a MySQL 5.7.9 compatible format tag while
the log blocks (other than the header and the checkpoint blocks)
are in an incompatible, encrypted format.
That needs to be fixed by introducing a new redo log format tag that
indicates that the entire redo log is encrypted.

mysql-test/suite/encryption/r/innodb_encrypt_log_corruption.result

@@ -86,11 +86,10 @@ AND support IN ('YES', 'DEFAULT', 'ENABLED');
 ENGINE	SUPPORT	COMMENT	TRANSACTIONS	XA	SAVEPOINTS
 FOUND /len 22; hex 38000000000012860cb7809781e800066269676f7400; asc 8               bigot ;/ in mysqld.1.err
 # Minimal MariaDB 10.1.21 encrypted redo log
-# FIXME: Upgrade from a clean encrypted redo log should actually work!
 SELECT COUNT(*) `1` FROM INFORMATION_SCHEMA.ENGINES WHERE engine='innodb'
 AND support IN ('YES', 'DEFAULT', 'ENABLED');
 1
-0
+1
 ib_buffer_pool
 ib_logfile0
 ib_logfile1

mysql-test/suite/encryption/t/innodb_encrypt_log_corruption.test

 --let $no_cleanup=1
 --source ../../innodb/t/log_corruption.test
 
---echo # FIXME: Upgrade from a clean encrypted redo log should actually work!
-
 SELECT COUNT(*) `1` FROM INFORMATION_SCHEMA.ENGINES WHERE engine='innodb'
 AND support IN ('YES', 'DEFAULT', 'ENABLED');
 

storage/innobase/log/log0recv.cc

@@ -872,6 +872,8 @@ recv_log_format_0_recover(lsn_t lsn)
 			% univ_page_size.physical()),
 		OS_FILE_LOG_BLOCK_SIZE, buf, NULL);
 
+	log_decrypt_after_read(buf, OS_FILE_LOG_BLOCK_SIZE);
+
 	if (log_block_calc_checksum_format_0(buf)
 	    != log_block_get_checksum(buf)) {
 		ib::error() << NO_UPGRADE_RECOVERY_MSG