btrfs: dev-replace: remove unused members of btrfs_dev_replace

Lock owner and nesting level have been unused since day 1, probably
copy&pasted from the extent_buffer locking scheme without much thinking.
The locking of device replace is simpler and does not need any lock
nesting.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -365,8 +365,6 @@ struct btrfs_dev_replace {
 	struct btrfs_device *srcdev;
 	struct btrfs_device *tgtdev;
 
-	pid_t lock_owner;
-	atomic_t nesting_level;
 	struct mutex lock_finishing_cancel_unmount;
 	rwlock_t lock;
 	atomic_t read_locks;

fs/btrfs/disk-io.c

@@ -2154,8 +2154,6 @@ static void btrfs_init_btree_inode(struct btrfs_fs_info *fs_info)
 
 static void btrfs_init_dev_replace_locks(struct btrfs_fs_info *fs_info)
 {
-	fs_info->dev_replace.lock_owner = 0;
-	atomic_set(&fs_info->dev_replace.nesting_level, 0);
 	mutex_init(&fs_info->dev_replace.lock_finishing_cancel_unmount);
 	rwlock_init(&fs_info->dev_replace.lock);
 	atomic_set(&fs_info->dev_replace.read_locks, 0);