ceph-fuse: add option to disable kernel pagecache

ceph-fuse: add option to disable kernel pagecache

When multiple clients read/write to a file at the same time, we need
to disable cache and let reads/writes go to OSDs. In ceph-fuse case,
there are two caches, one is in ceph-fuse, another one is in kernel.
We can't disable/enable the kernel cache dynamically. So adding an
config option to completely disable kernel cache. Disabling kernel
cache may reduce performance, so option is disabled by default.

Fixes: 12653
Signed-off-by: Yan, Zheng <zyan@redhat.com>