mds: debugging output on logsegment destruction

If there are dentries still there on deletion (there shouldn't be)
then print them out before subsequently hitting the assertion
in ~elist.  Inspired by debugging #10368

This is a teardown() method instead of ~LogSegment so that
we can pass in the MDS* pointer that the dout prefix expects.

Signed-off-by: John Spray <john.spray@redhat.com>