bcache: use NULL instead of using plain integer as pointer

This fixes the following sparse warnings:
drivers/md/bcache/features.c:22:16: warning: Using plain integer as NULL
pointer

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20210411134316.80274-4-colyli@suse.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>