dcssblk: handle alloc_dax() -EOPNOTSUPP failure

In preparation for checking whether the architecture has data cache
aliasing within alloc_dax(), modify the error handling of dcssblk
dcssblk_add_store() to handle alloc_dax() -EOPNOTSUPP failures.

Considering that s390 is not a data cache aliasing architecture,
and considering that DCSSBLK selects DAX, a return value of -EOPNOTSUPP
from alloc_dax() should make dcssblk_add_store() fail.

Link: https://lkml.kernel.org/r/20240215144633.96437-6-mathieu.desnoyers@efficios.com
Fixes: d92576f1 ("dax: does not work correctly with virtual aliasing caches")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Cc: Alasdair Kergon <agk@redhat.com>
Cc: Mike Snitzer <snitzer@kernel.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: kernel test robot <lkp@intel.com>
Cc: Michael Sclafani <dm-devel@lists.linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/s390/block/dcssblk.c

@@ -549,6 +549,7 @@ dcssblk_add_store(struct device *dev, struct device_attribute *attr, const char
 	int rc, i, j, num_of_segments;
 	struct dcssblk_dev_info *dev_info;
 	struct segment_info *seg_info, *temp;
+	struct dax_device *dax_dev;
 	char *local_buf;
 	unsigned long seg_byte_size;
 
@@ -677,13 +678,13 @@ dcssblk_add_store(struct device *dev, struct device_attribute *attr, const char
 	if (rc)
 		goto put_dev;
 
-	dev_info->dax_dev = alloc_dax(dev_info, &dcssblk_dax_ops);
-	if (IS_ERR(dev_info->dax_dev)) {
-		rc = PTR_ERR(dev_info->dax_dev);
-		dev_info->dax_dev = NULL;
+	dax_dev = alloc_dax(dev_info, &dcssblk_dax_ops);
+	if (IS_ERR(dax_dev)) {
+		rc = PTR_ERR(dax_dev);
 		goto put_dev;
 	}
-	set_dax_synchronous(dev_info->dax_dev);
+	set_dax_synchronous(dax_dev);
+	dev_info->dax_dev = dax_dev;
 	rc = dax_add_host(dev_info->dax_dev, dev_info->gd);
 	if (rc)
 		goto out_dax;