mds: fix blocking in standby replay thread

mds: fix blocking in standby replay thread

We need to hold mylock before waiting on the cond or else we get

./common/Cond.h: In function 'int Cond::Wait(Mutex&)', in thread '7f37fe0c8700'
./common/Cond.h: 46: FAILED assert(mutex.is_locked())
 ceph version 0.38-2-g73f99a1 (commit:73f99a189f491866da2be88adcfe0bd512282755)
 1: (MDLog::_replay_thread()+0x2483) [0x6c4393]
 2: (MDLog::ReplayThread::entry()+0xd) [0x4decbd]
 3: (()+0x6d8c) [0x7f3803e8fd8c]
 4: (clone()+0x6d) [0x7f38028d504d]
 ceph version 0.38-2-g73f99a1 (commit:73f99a189f491866da2be88adcfe0bd512282755)
 1: (MDLog::_replay_thread()+0x2483) [0x6c4393]
 2: (MDLog::ReplayThread::entry()+0xd) [0x4decbd]
 3: (()+0x6d8c) [0x7f3803e8fd8c]
 4: (clone()+0x6d) [0x7f38028d504d]
*** Caught signal (Aborted) **
 in thread 7f37fe0c8700

Signed-off-by: Sage Weil <sage@newdream.net>