btrfs-progs: add commit_root_sem to btrfs_fs_info

This is used in ctree.c around getting the old root, add this to our
btrfs_fs_info to make it more straightforward to sync ctree.c into
btrfs-progs.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

kernel-shared/ctree.h

@@ -329,6 +329,7 @@ struct btrfs_fs_info {
 	struct extent_io_tree *excluded_extents;
 
 	spinlock_t trans_lock;
+	struct rw_semaphore commit_root_sem;
 
 	struct rb_root block_group_cache_tree;
 	/* logical->physical extent mapping */

kernel-shared/disk-io.c

@@ -879,6 +879,7 @@ struct btrfs_fs_info *btrfs_new_fs_info(int writable, u64 sb_bytenr)
 	INIT_LIST_HEAD(&fs_info->recow_ebs);
 
 	spin_lock_init(&fs_info->trans_lock);
+	init_rwsem(&fs_info->commit_root_sem);
 
 	if (!writable)
 		fs_info->readonly = 1;