block/partition: fix signedness issue for Amiga partitions

Making 'blk' sector_t (i.e. 64 bit if LBD support is active) fails the
'blk>0' test in the partition block loop if a value of (signed int) -1 is
used to mark the end of the partition block list.

Explicitly cast 'blk' to signed int to allow use of -1 to terminate the
partition block linked list.

Fixes: b6f3f28 ("block: add overflow checks for Amiga partition support")
Reported-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Link: https://lore.kernel.org/r/024ce4fa-cc6d-50a2-9aae-3701d0ebf668@xenosoft.de
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Reviewed-by: Martin Steigerwald <martin@lichtvoll.de>
Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Michael Schmitz

block/partitions/amiga.c

@@ -90,7 +90,7 @@ int amiga_partition(struct parsed_partitions *state)
 	}
 	blk = be32_to_cpu(rdb->rdb_PartitionList);
 	put_dev_sector(sect);
-	for (part = 1; blk>0 && part<=16; part++, put_dev_sector(sect)) {
+	for (part = 1; (s32) blk>0 && part<=16; part++, put_dev_sector(sect)) {
 		/* Read in terms partition table understands */
 		if (check_mul_overflow(blk, (sector_t) blksize, &blk)) {
 			pr_err("Dev %s: overflow calculating partition block %llu! Skipping partitions %u and beyond\n",