mds: fix uninitialized fields in MDiscover

Fixes: #7739
Signed-off-by: Sage Weil <sage@inktank.com>
msgr: add ms_dump_on_send option

This is useful only for debugging.  The encoded contents of a message are
dumped to the log on message send.  This is useful when valgrind is
triggering warnings about uninitialized memory in messages because the
call chain will indicate which message type is to blame, whereas the
usual writer thread context does not tell us any useful information.

Signed-off-by: Sage Weil <sage@inktank.com>
mds: fix uninit MMDSSlaveRequest lock_type

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1609 from ceph/wip-7739

mds: fix some uninitialized message fields

Reviewed-by: Zheng Yan <zheng.z.yan@intel.com>