nfs: don't call bdi_unregister

bdi_destroy already does all the work, and if we delay freeing the
anon bdev we can get away with just that single call.

Addintionally remove the call during mount failure, as
deactivate_super_locked will already call ->kill_sb and clean up
the bdi for us.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>