Make compile successful when CONFIG_BCACHE is not set.

euleros inclusion
category: feature
bugzilla: https://bugzilla.openeuler.org/show_bug.cgi?id=327


CVE: NA

When kernel config don't enbale CONFIG_BCACHE, compiling bcache module will
fail. This patch add the judgment for CONFIG_BCACHE macro to make sure
compiling bcache module success.

Signed-off-by: qinghaixiang <xuweiqhx@163.com>
Signed-off-by: Xu Wei <xuwei56@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>
Reviewed-by: Li Ruilin <liruilin4@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>