s390/cio: Fix device lifecycle handling in css_alloc_subchannel()

`css_alloc_subchannel()` calls `device_initialize()` before setting up
the DMA masks. If `dma_set_coherent_mask()` or `dma_set_mask()` fails,
the error path frees the subchannel structure directly, bypassing
the device model reference counting.

Once `device_initialize()` has been called, the embedded struct device
must be released via `put_device()`, allowing the release callback to
free the container structure.

Fix the error path by dropping the initial device reference with
`put_device()` instead of calling `kfree()` directly.

This ensures correct device lifetime handling and avoids potential
use-after-free or double-free issues.

Fixes: e5dcf00 ("s390/css: move subchannel lock allocation")
Signed-off-by: Salah Triki <salah.triki@gmail.com>
Reviewed-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Author: salah-triki

drivers/s390/cio/css.c

@@ -235,7 +235,7 @@ struct subchannel *css_alloc_subchannel(struct subchannel_id schid,
 	return sch;
 
 err:
-	kfree(sch);
+	put_device(&sch->dev);
 	return ERR_PTR(ret);
 }