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

fs/btrfs/extent-tree.c

@@ -8464,7 +8464,6 @@ static noinline void reada_walk_down(struct btrfs_trans_handle *trans,
 	u64 refs;
 	u64 flags;
 	u32 nritems;
-	u32 blocksize;
 	struct btrfs_key key;
 	struct extent_buffer *eb;
 	int ret;
@@ -8482,7 +8481,6 @@ static noinline void reada_walk_down(struct btrfs_trans_handle *trans,
 
 	eb = path->nodes[wc->level];
 	nritems = btrfs_header_nritems(eb);
-	blocksize = root->nodesize;
 
 	for (slot = path->slots[wc->level]; slot < nritems; slot++) {
 		if (nread >= wc->reada_count)