ceph: metrics for opened files, pinned caps and opened inodes

In client for each inode, it may have many opened files and may
have been pinned in more than one MDS servers. And some inodes
are idle, which have no any opened files.

This patch will show these metrics in the debugfs, likes:

item                               total
-----------------------------------------
opened files  / total inodes       14 / 5
pinned i_caps / total inodes       7  / 5
opened inodes / total inodes       3  / 5

Will send these metrics to ceph, which will be used by the `fs top`,
later.

[ jlayton: drop unrelated hunk, count hashed inodes instead of
           allocated ones ]

URL: https://tracker.ceph.com/issues/47005


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>