btrfs: always use right leaf variable in __push_leaf_left()

btrfs: always use right leaf variable in __push_leaf_left()

The 'right' variable points to path->nodes[0] and path->nodes[0] is never
changed, but some places use 'right' while others refer to path->nodes[0].
Update all sites to use 'right' as not only it's shorter it's also easier
to reason since it means the right leaf and avoids any confusion with the
sibling left leaf.

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>