mds: Clear backtrace updates on standby_trim_seg

If the mds is standby, when a segment is trimmed, we need
to clear the backtrace updates list to avoid the following
assertion when the segment is deleted.

./include/elist.h: 92: FAILED assert(_head.empty())
ceph version 0.59-478-g8befbca (8befbca77aa50a1188969892aabedaf11d8f8ce7)
(MDLog::standby_trim_segments()+0xce5) [0x6ccec5]
(MDS::C_MDS_StandbyReplayRestartFinish::finish(int)+0x39) [0x4e86b9]
(Journaler::_finish_reprobe(int, unsigned long, Context*)+0x190)
[0x6d3210]
(Filer::_probed(Filer::Probe*, object_t const&, unsigned long,
utime_t)+0x558) [0x704a88]
(Objecter::C_Stat::finish(int)+0xc0) [0x705900]
(Objecter::handle_osd_op_reply(MOSDOpReply*)+0xe38) [0x6f1df8]
(MDS::handle_core_message(Message*)+0xae8) [0x4dc318]
(MDS::_dispatch(Message*)+0x2f) [0x4dc4df]
(MDS::ms_dispatch(Message*)+0x1db) [0x4ddf7b]
(DispatchQueue::entry()+0x341) [0x81f561]
(DispatchQueue::DispatchThread::entry()+0xd) [0x79c6ad]
(()+0x7e9a) [0x7f346bb9ee9a]
(clone()+0x6d) [0x7f346a3574bd]

Fixes #4539.
Signed-off-by: Sam Lang <sam.lang@inktank.com>