btrfs: use a local variable for fs_devices pointer in btrfs_dev_replace_finishing

In the function btrfs_dev_replace_finishing, we dereferenced
fs_info->fs_devices 6 times. Use keep local variable for that.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>