s390/dcssblk: fix kernel crash with list_add corruption

In the Linux kernel, the following vulnerability has been resolved:

s390/dcssblk: fix kernel crash with list_add corruption

Commit fb08a1908cb1 ("dax: simplify the dax_device <-> gendisk

association") introduced new logic for gendisk association, requiring

drivers to explicitly call dax_add_host() and dax_remove_host().

For dcssblk driver, some dax_remove_host() calls were missing, e.g. in

device remove path. The commit also broke error handling for out_dax case

in device add path, resulting in an extra put_device() w/o the previous

get_device() in that case.

This lead to stale xarray entries after device add / remove cycles. In the

case when a previously used struct gendisk pointer (xarray index) would be

used again, because blk_alloc_disk() happened to return such a pointer, the

xa_insert() in dax_add_host() would fail and go to out_dax, doing the extra

put_device() in the error path. In combination with an already flawed error

handling in dcssblk (device_register() cleanup), which needs to be

addressed in a separate patch, this resulted in a missing device_del() /

klist_del(), and eventually in the kernel crash with list_add corruption on

a subsequent device_add() / klist_add().

Fix this by adding the missing dax_remove_host() calls, and also move the

put_device() in the error path to restore the previous logic.