dm thin metadata: Fix use-after-free in dm_bm_set_read_only

The following error ocurred when testing disk online/offline:

[  301.798344] device-mapper: thin: 253:5: aborting current metadata transaction
[  301.848441] device-mapper: thin: 253:5: failed to abort metadata transaction
[  301.849206] Aborting journal on device dm-26-8.
[  301.850489] EXT4-fs error (device dm-26) in __ext4_new_inode:943: Journal has aborted
[  301.851095] EXT4-fs (dm-26): Delayed block allocation failed for inode 398742 at logical offset 181 with max blocks 19 with error 30
[  301.854476] BUG: KASAN: use-after-free in dm_bm_set_read_only+0x3a/0x40 [dm_persistent_data]

Reason is:

 metadata_operation_failed
    abort_transaction
        dm_pool_abort_metadata
	    __create_persistent_data_objects
	        r = __open_or_format_metadata
	        if (r) --> If failed will free pmd->bm but pmd->bm not set NULL
		    dm_block_manager_destroy(pmd->bm);
    set_pool_mode
	dm_pool_metadata_read_only(pool->pmd);
	dm_bm_set_read_only(pmd->bm);  --> use-after-free

Add checks to see if pmd->bm is NULL in dm_bm_set_read_only and
dm_bm_set_read_write functions.  If bm is NULL it means creating the
bm failed and so dm_bm_is_read_only must return true.

Signed-off-by: Ye Bin <yebin10@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>