btrfs: clean up locking name in scrub_enumerate_chunks()

The "&fs_info->dev_replace.rwsem" and "&dev_replace->rwsem" refer to
the same lock but Smatch is not clever enough to figure that out so it
leads to static checker warnings.  It's better to use it consistently
anyway.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>