btrfs: return void from readahead_tree_block

Errors in readahead are not fatal and ignored elsewhere in the code.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h
index 8cd6a53..0d9793f 100644
--- a/fs/btrfs/disk-io.h
+++ b/fs/btrfs/disk-io.h
@@ -46,7 +46,7 @@
 
 struct extent_buffer *read_tree_block(struct btrfs_root *root, u64 bytenr,
 				      u32 blocksize, u64 parent_transid);
-int readahead_tree_block(struct btrfs_root *root, u64 bytenr, u32 blocksize);
+void readahead_tree_block(struct btrfs_root *root, u64 bytenr, u32 blocksize);
 int reada_tree_block_flagged(struct btrfs_root *root, u64 bytenr, u32 blocksize,
 			 int mirror_num, struct extent_buffer **eb);
 struct extent_buffer *btrfs_find_create_tree_block(struct btrfs_root *root,