This fixes several InnoDB bugs related to innodb_encrypt_log and
two Mariabackup --backup bugs.

log_crypt(): Properly derive the initialization vector from the
start LSN of each block. Add a debug assertion.

log_crypt_init(): Note that the function should only be used when
creating redo log files and that the information is persisted in
the checkpoint pages.

xtrabackup_copy_log(): Validate data_len.

xtrabackup_backup_func(): Always use the chosen checkpoint buffer.

log_group_write_buf(), log_write_up_to(): Only log_crypt() the redo
log payload, not the padding bytes.

innobase_start_or_create_for_mysql(): Do not invoke log_crypt_init()
or initiate a redo log checkpoint.

recv_find_max_checkpoint(): Return the contents of LOG_CHECKPOINT_NO
to xtrabackup_backup_func() in log_sys->next_checkpoint_no.