mds: store backtrace for straydir

Backport: giant, firefly, emperor, dumpling
Signed-off-by: Yan, Zheng <zyan@redhat.com>
(cherry picked from commit 0d89db5d3e5ae5d552d4058a88a4e186748ab1d2)
Merge pull request #3107 from ceph/emperor-10263

mds: store backtrace for straydir

Reviewed-by: Greg Farnum <gfarnum@redhat.com>