btrfs: remove variable to track trimmed bytes at btrfs_finish_extent_commit()

btrfs: remove variable to track trimmed bytes at btrfs_finish_extent_commit()

We don't need to keep track of discarded (trimmed) bytes at
btrfs_finish_extent_commit() but we are declaring a local variable for
that and passing a reference to the btrfs_discard_extent() calls when we
are processing delete block groups. So instead pass NULL to
btrfs_discard_extent() and remove that variable.

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