Btrfs: record first logical byte in memory

This'd save us a rbtree search which may become expensive in large filesystem.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/disk-io.c

@@ -2130,6 +2130,7 @@ int open_ctree(struct super_block *sb,
 
 	spin_lock_init(&fs_info->block_group_cache_lock);
 	fs_info->block_group_cache_tree = RB_ROOT;
+	fs_info->first_logical_byte = (u64)-1;
 
 	extent_io_tree_init(&fs_info->freed_extents[0],
 			     fs_info->btree_inode->i_mapping);