dm verity: initialize fec io before freeing it

If BIO error, verity_end_io() can call verity_finish_io() before
verity_fec_init_io(). Therefore, fec_io->rs is not initialized and
may crash when doing memory freeing in verity_fec_finish_io().

Crash call stack:
 die+0x90/0x2b8
 __do_kernel_fault+0x260/0x298
 do_bad_area+0x2c/0xdc
 do_translation_fault+0x3c/0x54
 do_mem_abort+0x54/0x118
 el1_abort+0x38/0x5c
 el1h_64_sync_handler+0x50/0x90
 el1h_64_sync+0x64/0x6c
 free_rs+0x18/0xac
 fec_rs_free+0x10/0x24
 mempool_free+0x58/0x148
 verity_fec_finish_io+0x4c/0xb0
 verity_end_io+0xb8/0x150

Cc: [email protected]      # v6.0+
Fixes: 5721d4e ("dm verity: Add optional "try_verify_in_tasklet" feature")
Signed-off-by: Wu Bo <[email protected]>
Reviewed-by: Mikulas Patocka <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>

Author: Wu Bo

drivers/md/dm-verity-target.c

@@ -642,7 +642,6 @@ static void verity_work(struct work_struct *w)
 
 	io->in_tasklet = false;
 
-	verity_fec_init_io(io);
 	verity_finish_io(io, errno_to_blk_status(verity_verify_io(io)));
 }
 
@@ -792,6 +791,8 @@ static int verity_map(struct dm_target *ti, struct bio *bio)
 	bio->bi_private = io;
 	io->iter = bio->bi_iter;
 
+	verity_fec_init_io(io);
+
 	verity_submit_prefetch(v, io);
 
 	submit_bio_noacct(bio);