blk_getlength() returns an int64_t but the result is stored in a
uint32_t. Errors (negative values) won't be caught by the check in
pnv_pnor_realize() and blk_blockalign() will allocate a very large
buffer in such cases.

Fixes Coverity issue CID 1412226.
Signed-off-by: NCédric Le Goater <clg@kaod.org>
Message-Id: <20200107171809.15556-3-clg@kaod.org>
Reviewed-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>