LU-911 osd: couple helpers for osd api

LU-911 osd: couple helpers for osd api

- dt_find_or_create() to "open" and create if needed an object
- dt_object_sync() - a wrapper for corresponding osd api method

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: If10e2f522f6e5383260a1d729f305ccdc0b02bbb
Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1838
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>