md: disable bcache

It uses anon semaphores
|drivers/md/bcache/request.c: In function ‘cached_dev_write_complete’:
|drivers/md/bcache/request.c:1007:2: error: implicit declaration of function ‘up_read_non_owner’ [-Werror=implicit-function-declaration]
|  up_read_non_owner(&dc->writeback_lock);
|  ^
|drivers/md/bcache/request.c: In function ‘request_write’:
|drivers/md/bcache/request.c:1033:2: error: implicit declaration of function ‘down_read_non_owner’ [-Werror=implicit-function-declaration]
|  down_read_non_owner(&dc->writeback_lock);
|  ^

either we get rid of those or we have to introduce them…

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>