btrfs: Fix warning "variable ‘blocksize’ set but not used"

Variable 'blocksize' in reada_walk_down() is not used since commit
d3e46fea ("btrfs: sink blocksize parameter to readahead_tree_block").
This patch simply removes this variable.

Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>