ceph_fuse: make it to force invalidating dentries when kernel >=3.18

ceph_fuse: make it to force invalidating dentries when kernel >=3.18

The remount will fail randomly for unknown reasons. And in certain
circumstance we can reprodce this very easy, which will block our
testing. Make it posible to force to old method to invalidate the
dcache when the "client_try_dentry_invalidate" option is enabled
even kernel version >= 3.18.0

Fixes: https://tracker.ceph.com/issues/56532
Signed-off-by: Xiubo Li <xiubli@redhat.com>