client: invalidate kernel dentries one by one

Our trick to trim the whole kernel dentry tree does not work for 3.18+ kernel.
The fix is trim kernel dentries one by one.

Fixes: #9997
Signed-off-by: Yan, Zheng <zyan@redhat.com>
Merge pull request #3056 from ceph/wip-9997-1

client: invalidate kernel dentries one by one

Reviewed-by: John Spray <john.spray@redhat.com>