null_blk: fix spurious IO errors after failed past-wp access

Steps to reproduce:

	BLKRESETZONE zone 0

	// force EIO
	pwrite(fd, buf, 4096, 4096);

	[issue more IO including zone ioctls]

It will start failing randomly including IO to unrelated zones because of
->error "reuse". Trigger can be partition detection as well if test is not
run immediately which is even more entertaining.

The fix is of course to clear ->error where necessary.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexey Dobriyan (SK hynix) <adobriyan@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>