client: drop null child dentries before try pruning inode's alias

client: drop null child dentries before try pruning inode's alias

Null child dentries holds reference on inode's alias, they prevents
Client::trim_caps() from trimming the inode. Null dentries are trimmed
by Client::trim_cache() according to 'client_cache_size' config option.
So Client::trim_caps() may fail to trim as many caps as MDS asked when
client cache size is smaller than the config limit.

Fixes: http://tracker.ceph.com/issues/22504
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
(cherry picked from commit 99b1b959e13219dee10f5649be1c1c15e25f9ccd)