Partitions are not limited to live within a device.  So
we should range check after partition mapping.

Note that 'maxsector' was being used for two different things.  I have
split off the second usage into 'old_sector' so that maxsector can be
still be used for it's primary usage later in the function.

Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>