block: fix potential invalid pointer dereference in blk_add_partition

The blk_add_partition() function initially used a single if-condition
(IS_ERR(part)) to check for errors when adding a partition. This was
modified to handle the specific case of -ENXIO separately, allowing the
function to proceed without logging the error in this case. However,
this change unintentionally left a path where md_autodetect_dev()
could be called without confirming that part is a valid pointer.

This commit separates the error handling logic by splitting the
initial if-condition, improving code readability and handling specific
error scenarios explicitly. The function now distinguishes the general
error case from -ENXIO without altering the existing behavior of
md_autodetect_dev() calls.

Fixes: b7205307 (block: allow partitions on host aware zone devices)
Signed-off-by: Riyan Dhiman <riyandhiman14@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240911132954.5874-1-riyandhiman14@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/partitions/core.c

@@ -555,9 +555,11 @@ static bool blk_add_partition(struct gendisk *disk,
 
 	part = add_partition(disk, p, from, size, state->parts[p].flags,
 			     &state->parts[p].info);
-	if (IS_ERR(part) && PTR_ERR(part) != -ENXIO) {
-		printk(KERN_ERR " %s: p%d could not be added: %pe\n",
-		       disk->disk_name, p, part);
+	if (IS_ERR(part)) {
+		if (PTR_ERR(part) != -ENXIO) {
+			printk(KERN_ERR " %s: p%d could not be added: %pe\n",
+			       disk->disk_name, p, part);
+		}
 		return true;
 	}