s390/dasd: fix error checks in dasd_copy_pair_store()

dasd_add_busid() can return an error via ERR_PTR() if an allocation
fails. However, two callsites in dasd_copy_pair_store() do not check
the result, potentially resulting in a NULL pointer dereference. Fix
this by checking the result with IS_ERR() and returning the error up
the stack.

Fixes: a91ff09d ("s390/dasd: add copy pair setup")
Signed-off-by: Carlos López <clopez@suse.de>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://lore.kernel.org/r/20240715112434.2111291-3-sth@linux.ibm.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/s390/block/dasd_devmap.c

@@ -2248,13 +2248,19 @@ static ssize_t dasd_copy_pair_store(struct device *dev,
 
 	/* allocate primary devmap if needed */
 	prim_devmap = dasd_find_busid(prim_busid);
-	if (IS_ERR(prim_devmap))
+	if (IS_ERR(prim_devmap)) {
 		prim_devmap = dasd_add_busid(prim_busid, DASD_FEATURE_DEFAULT);
+		if (IS_ERR(prim_devmap))
+			return PTR_ERR(prim_devmap);
+	}
 
 	/* allocate secondary devmap if needed */
 	sec_devmap = dasd_find_busid(sec_busid);
-	if (IS_ERR(sec_devmap))
+	if (IS_ERR(sec_devmap)) {
 		sec_devmap = dasd_add_busid(sec_busid, DASD_FEATURE_DEFAULT);
+		if (IS_ERR(sec_devmap))
+			return PTR_ERR(sec_devmap);
+	}
 
 	/* setting copy relation is only allowed for offline secondary */
 	if (sec_devmap->device)