btrfs: cleanup the bytenr usage inside btrfs_extent_item_to_extent_map()

[HICCUP]
Before commit 85de2be7129c ("btrfs: remove extent_map::block_start
member"), we utilized @bytenr variable inside
btrfs_extent_item_to_extent_map() to calculate block_start.

But that commit removed block_start completely, we have no need to
advance @bytenr at all.

[ENHANCEMENT]
- Rename @bytenr as @disk_bytenr
- Only declare @disk_bytenr inside the if branch
- Make @disk_bytenr const and remove the modification on it

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>