dm thin metadata: check fail_io before using data_sm

Must check pmd->fail_io before using pmd->data_sm since
pmd->data_sm may be destroyed by other processes.

       P1(kworker)                             P2(message)
do_worker
 process_prepared
  process_prepared_discard_passdown_pt2
   dm_pool_dec_data_range
                                    pool_message
                                     commit
                                      dm_pool_commit_metadata
                                        ↓
                                       // commit failed
                                      metadata_operation_failed
                                       abort_transaction
                                        dm_pool_abort_metadata
                                         __open_or_format_metadata
                                           ↓
                                          dm_sm_disk_open
                                            ↓
                                           // open failed
                                           // pmd->data_sm is NULL
    dm_sm_dec_blocks
      ↓
     // try to access pmd->data_sm --> UAF

As shown above, if dm_pool_commit_metadata() and
dm_pool_abort_metadata() fail in pool_message process, kworker may
trigger UAF.

Fixes: be500ed7

 ("dm space maps: improve performance with inc/dec on ranges of blocks")
Cc: stable@vger.kernel.org
Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>