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: b6f3f28f ("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>