messages/MOSDMarkMeDown: fix uninit field

Fixes valgrind warning:
==14803== Use of uninitialised value of size 8
==14803==    at 0x12E7614: sctp_crc32c_sb8_64_bit (sctp_crc32.c:567)
==14803==    by 0x12E76F8: update_crc32 (sctp_crc32.c:609)
==14803==    by 0x12E7720: ceph_crc32c_le (sctp_crc32.c:733)
==14803==    by 0x105085F: ceph::buffer::list::crc32c(unsigned int) (buffer.h:427)
==14803==    by 0x115D7B2: Message::calc_front_crc() (Message.h:441)
==14803==    by 0x1159BB0: Message::encode(unsigned long, bool) (Message.cc:170)
==14803==    by 0x1323934: Pipe::writer() (Pipe.cc:1524)
==14803==    by 0x13293D9: Pipe::Writer::entry() (Pipe.h:59)
==14803==    by 0x120A398: Thread::_entry_func(void*) (Thread.cc:41)
==14803==    by 0x503BE99: start_thread (pthread_create.c:308)
==14803==    by 0x6C6E4BC: clone (clone.S:112)

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>