dm: cope with access beyond end of device in dm_merge_bvec

If for any reason dm_merge_bvec() is given an offset beyond the end of the
device, avoid an oops and always allow one page to be added to an empty bio.
We'll reject the I/O later after the bio is submitted.

Signed-off-by: Mikulas Patocka <[e-mail]>
Signed-off-by: Alasdair G Kergon <[e-mail]>

drivers/md/dm.c

···
 		goto out;
 
 	ti = dm_table_find_target(map, bvm->bi_sector);
+	if (!dm_target_is_valid(ti))
+		goto out_table;
 
 	/*
 	 * Find maximum amount of I/O that won't need splitting
···
 	if (max_size && ti->type->merge)
 		max_size = ti->type->merge(ti, bvm, biovec, max_size);
 
+out_table:
 	dm_table_put(map);
 
 out: