mds: batch backtrace updates by pool-id when expiring a log segment

mds: batch backtrace updates by pool-id when expiring a log segment

LogSegment::try_to_expire() batches backtrace updations for inodes in
dirty_parent_inodes list. If a backtrace update operations fails for
one inode due to missing (removed) data pool, which is specially
handled by treating the operation as a success, however, the errno
(-ENOENT) is stored by the gather context and passed on as the return
value to subsequent operations (even for successful backtrace update
operations in the same gather context).

Fixes: http://tracker.ceph.com/issues/63259
Signed-off-by: Venky Shankar <vshankar@redhat.com>
(cherry picked from commit d1d3a8c7c80534cba26d1daf2f60f4c7f03c8339)