MAINTAINERS: mark bcache as orphan

The submitted patches are not being reacted upon, and Jens is only picking
up stable fixes on an rather ad-hoc basis.

Link: lkml.kernel.org/r/574462C5.40307@kernel.dk
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>