nvme: fix max_discard_sectors calculation

ctrl->max_discard_sectors stores a value that is potentially based of
the DMRSL field in Identify Controller, which is in units of LBAs and
thus dependent on the Format of a namespace.

Fix this by moving the calculation of max_discard_sectors entirely
into nvme_config_discard and replacing the ctrl->max_discard_sectors
value with a local variable so that the calculation is always
namespace-specific.

Fixes: 1a86924e ("nvme: fix interpretation of DMRSL")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>