mds: add "mds traceless replies" debug option

This option specifies (in the range 0-1) the percentage of modifying
operations that should be responded to without including a trace
for the dentries.
In order to implement it, we add a "modify" parameter to
set_trace_dist(), and the callers fill that with
mdr->req->may_write().

Signed-off-by: Greg Farnum <greg@inktank.com>