btrfs: always use left leaf variable in __push_leaf_right()

btrfs: always use left leaf variable in __push_leaf_right()

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