ceph: add trace points to the MDS client

ceph: add trace points to the MDS client

This patch adds trace points to the Ceph filesystem MDS client:

- request submission (CEPH_MSG_CLIENT_REQUEST) and completion
  (CEPH_MSG_CLIENT_REPLY)
- capabilities (CEPH_MSG_CLIENT_CAPS)

These are the central pieces that are useful for analyzing MDS
latency/performance problems from the client's perspective.

In the long run, all doutc() calls should be replaced with
tracepoints.  This way, the Ceph filesystem can be traced at any time
(without spamming the kernel log).  Additionally, trace points can be
used in BPF programs (which can even deference the pointer parameters
and extract more values).

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>