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 Client:trim_dentry() from unlinking parent dentry of
directory inode referenced by fuse kernel module.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1606 from ceph/wip-shrink-icache

client: try shrinking kernel inode cache when trimming session caps

Reviewed-by: Sage Weil <sage@inktank.com>