common/EventTrace: fix compiler warning.

common/EventTrace: fix compiler warning.

/mnt/ceph/src/osd/OSD.cc: In member function ‘void
OSD::dequeue_op(PGRef, OpRequestRef, ThreadPool::TPHandle&)’:
/mnt/ceph/src/osd/OSD.cc:9112:42: error: invalid conversion from ‘const
Message*’ to ‘Message*’ [-fpermissive]
   OID_EVENT_TRACE_WITH_MSG(op->get_req(), "DEQUEUE_OP_BEGIN", false);
                                          ^
In file included from /mnt/ceph/src/osd/OSD.h:57:0,
                 from /mnt/ceph/src/osd/OSD.cc:38:
/mnt/ceph/src/common/EventTrace.h:72:15: note:   initializing argument 1
of ‘static void EventTrace::trace_oid_event(Message*, const char*, const
char*, const char*, int, bool)’
   static void trace_oid_event(Message *m, const char *event, const char
*file,
               ^
/mnt/ceph/src/osd/OSD.cc:9158:42: error: invalid conversion from ‘const
Message*’ to ‘Message*’ [-fpermissive]
   OID_EVENT_TRACE_WITH_MSG(op->get_req(), "DEQUEUE_OP_END", true);
                                          ^
In file included from /mnt/ceph/src/osd/OSD.h:57:0,
                 from /mnt/ceph/src/osd/OSD.cc:38:
/mnt/ceph/src/common/EventTrace.h:72:15: note:   initializing argument 1
of ‘static void EventTrace::trace_oid_event(Message*, const char*, const
char*, const char*, int, bool)’
   static void trace_oid_event(Message *m, const char *event, const char
*file,
               ^
src/osd/CMakeFiles/osd.dir/build.make:62: recipe for target
'src/osd/CMakeFiles/osd.dir/OSD.cc.o' failed

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>