client: try shrinking kernel inode cache when trimming session caps

Notify kernel to invalidate top level directory entries. As a side
effect, the kernel inode cache get shrinked.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
client: pin parent dentry of inode who has ll_ref > 0

This prevents directory inodes referenced by fuse kernel modules from
being detached from fs hierarchy.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>