btrfs: dev-replace: remove custom read/write blocking scheme

After the rw semaphore has been added, the custom blocking using
::blocking_readers and ::read_lock_wq is redundant.

The blocking logic in __btrfs_map_block is replaced by extending the
time the semaphore is held, that has the same blocking effect on writes
as the previous custom scheme that waited until ::blocking_readers was
zero.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -379,8 +379,6 @@ struct btrfs_dev_replace {
 
 	struct mutex lock_finishing_cancel_unmount;
 	struct rw_semaphore rwsem;
-	atomic_t blocking_readers;
-	wait_queue_head_t read_lock_wq;
 
 	struct btrfs_scrub_progress scrub_progress;