Ceph » CephFS

PR #22555 (https://github.com/ceph/ceph/pull/22555/) converted all Message handling to use const to avoid potential errors with re-processing messages that have been changed elsewhere or (in the future) by another thread.

There were a few instances which were not resolved in the interest of getting #22555 done:

~/ceph/src/messages$ git grep mutable
MClientRequest.h:  mutable struct ceph_mds_request_head head; /* XXX HACK! */
MClientRequest.h:    mutable ceph_mds_request_release item;
MClientRequest.h:  mutable vector<Release> releases; /* XXX HACK! */
MClientRequest.h:  mutable bool queued_for_replay = false;
MDirUpdate.h:  mutable int tried_discover = 0; // XXX HACK
MMDSSlaveRequest.h:  mutable __u16 flags; /* XXX HACK for mark_interrupted */
MMDSSlaveRequest.h:  mutable bufferlist straybl;  // stray dir + dentry

Mostly, these are state the MDS uses when reprocessing a Message (after acquiring locks for instance). The MDS should split this state off into a Context and keep the Message purely const.