ReplicatedPG: lock snapdir obc during write

Otherwise, we won't block properly in prep_push_backfill_object.

Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit b87bc2311aa4da065477f402a869e2edc1558e2f)

Conflicts:
	src/osd/ReplicatedPG.h
Merge pull request #1931 from ceph/wip-7068-dumpling

Wip 7068 dumpling

Reviewed-by: Sage Weil <sage@inktank.com>