new helper - update_partition(disk, partition_number); does the
right thing wrt devfs and driverfs (un)registration of partition entries.
BLKPG ioctls fixed - now they call that beast rather than calling only
devfs side.  New helper - rescan_partitions(disk, bdev); does all work
with wiping/rereading/etc. and fs/block_dev.c now uses it instead of
check_partition().  The latter became static.