btrfs: use local key variable to pass arguments in replay_one_extent()

Instead of extracting again the disk_bytenr and disk_num_bytes values from
the file extent item to pass to btrfs_qgroup_trace_extent(), use the key
local variable 'ins' which already has those values, reducing the size of
the source code.

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