mds: rename mds_traceless_replies to mds_inject_traceless_reply_probability

Sage pointed out we should try for consistent naming on these debug
options, and this option is like our other inject_[x] stuff.
While we're here, make the config option comment a bit clearer.

08b82b3ef6b43283e35fd4e56eb5c78651345bea
Merge git://github.com/ceph/ceph