btrfs: re-introduce struct btrfs_io_geometry

Re-introduce struct btrfs_io_geometry, holding the necessary bits and
pieces needed in btrfs_map_block() to decide the I/O geometry of a specific
block mapping.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>