PXB-3762: xtrabackup --backup --transition-key crashes when undo tablespace key exists in redo only

JIRA URL: https://perconadev.atlassian.net/browse/PXB-3762

The encryption_info keys map has two phases:
  - --backup  : producer. Keys are inserted from redo and dumped to
                xtrabackup_keys at end-of-backup.
  - --prepare : consumer. xtrabackup_keys is loaded into the map and
                looked up to set up encryption on each tablespace.

xb_set_encryption() / xb_fetch_tablespace_key() are the consumer side
and must only run at prepare time. Condition 2 in
srv_undo_tablespace_read_encryption() gates this correctly with
(use_dumped_tablespace_keys && !srv_backup_mode); Condition 1 was
missing the !srv_backup_mode half.

--transition-key sets use_dumped_tablespace_keys=true in both phases,
so when redo carried an undo-encryption key newer than the on-disk
undo page, --backup entered the consumer path against an empty map
and ut_a(found) fired.

Add !srv_backup_mode to Condition 1 to match Condition 2. During
backup we now use the key already in recv_key (which is what gets
inserted into the map anyway).

Author: JoeJRW

storage/innobase/srv/srv0start.cc

@@ -392,7 +392,7 @@ static dberr_t srv_undo_tablespace_read_encryption(pfs_os_file_t fh,
 
   if (found && recv_key->lsn >= page_lsn) {
     // Condition 1: Use key from redo if available and appropriate
-    encryption_success = use_dumped_tablespace_keys
+    encryption_success = (use_dumped_tablespace_keys && !srv_backup_mode)
                              ? load_key_from_dump()
                              : set_encryption(recv_key->ptr, recv_key->iv);
   } else if (is_enc) {