mds: Keep LogSegment ref for openc backtrace

The MDRequest is destroyed once the client reply is sent, but
we need the reference to the LogSegment for updating the backtrace, so
store a temporary ref to the LogSegment for later.

Fixes #4660.
Signed-off-by: Sam Lang <sam.lang@inktank.com>
Merge pull request #206 from ceph/wip-4660

mds: Keep LogSegment ref for openc backtrace

Reviewed-by: Greg Farnum <greg@inktank.com>