btrfs: simplify inline extent end calculation at replay_one_extent()

There is no need to store the extent's ram_bytes in two variables,
further more one of them, named 'size', is used only for the extent's end
offset calculation. So remove the 'size' variable and use 'nbytes' only.

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