md/md-linear: Fix a NULL vs IS_ERR() bug in linear_add()

md/md-linear: Fix a NULL vs IS_ERR() bug in linear_add()

commit 62c552070a980363d55a6082b432ebd1cade7a6e upstream.

The linear_conf() returns error pointers, it doesn't return NULL.  Update
the error checking to match.

Fixes: 127186cfb184 ("md: reintroduce md-linear")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/add654be-759f-4b2d-93ba-a3726dae380c@stanley.mountain
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>