register_shrinker is now __must_check, so check it to kill a warning.
Caller of bch_btree_cache_alloc in super.c appropriately checks return
value so this is fully plumbed through.

This V2 fixes checkpatch warnings and improves the commit description,
as I was too hasty getting the previous version out.
Signed-off-by: NMichael Lyle <mlyle@lyle.org>
Reviewed-by: NVojtech Pavlik <vojtech@suse.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>