fs: Remove redundant errseq_set call in mark_buffer_write_io_error.

mark_buffer_write_io_error sets sb->s_wb_err to -EIO twice.
Once in mapping_set_error and once in errseq_set.
Only mapping_set_error checks if bh->b_assoc_map->host is NULL.

Discovered during null pointer dereference during writeback
to a failing device:

[<ffffffff9a416dc8>] ? mark_buffer_write_io_error+0x98/0xc0
[<ffffffff9a416dbe>] ? mark_buffer_write_io_error+0x8e/0xc0
[<ffffffff9ad4bda0>] end_buffer_async_write+0x90/0xd0
[<ffffffff9ad4e3eb>] end_bio_bh_io_sync+0x2b/0x40
[<ffffffff9adbafe6>] blk_update_request+0x1b6/0x480
[<ffffffff9adbb3d8>] blk_mq_end_request+0x18/0x30
[<ffffffff9adbc6aa>] blk_mq_dispatch_rq_list+0x4da/0x8e0
[<ffffffff9adc0a68>] __blk_mq_sched_dispatch_requests+0x218/0x6a0
[<ffffffff9adc07fa>] blk_mq_sched_dispatch_requests+0x3a/0x80
[<ffffffff9adbbb98>] blk_mq_run_hw_queue+0x108/0x330
[<ffffffff9adbcf58>] blk_mq_flush_plug_list+0x178/0x5f0
[<ffffffff9adb6741>] __blk_flush_plug+0x41/0x120
[<ffffffff9adb6852>] blk_finish_plug+0x22/0x40
[<ffffffff9ad47cb0>] wb_writeback+0x150/0x280
[<ffffffff9ac5343f>] ? set_worker_desc+0x9f/0xc0
[<ffffffff9ad4676e>] wb_workfn+0x24e/0x4a0

Fixes: 485e960 ("fs/buffer.c: record blockdev write errors in super_block that it backs")
Signed-off-by: Jeremy Bongio <jbongio@google.com>
Link: https://lore.kernel.org/20250507123010.1228243-1-jbongio@google.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: bongiojp

fs/buffer.c

@@ -1220,10 +1220,8 @@ void mark_buffer_write_io_error(struct buffer_head *bh)
 	/* FIXME: do we need to set this in both places? */
 	if (bh->b_folio && bh->b_folio->mapping)
 		mapping_set_error(bh->b_folio->mapping, -EIO);
-	if (bh->b_assoc_map) {
+	if (bh->b_assoc_map)
 		mapping_set_error(bh->b_assoc_map, -EIO);
-		errseq_set(&bh->b_assoc_map->host->i_sb->s_wb_err, -EIO);
-	}
 }
 EXPORT_SYMBOL(mark_buffer_write_io_error);