LU-911 osd: fix th_sync and OSD_EXEC_OP issues

This issues occurs during the porting from Orion to the master
- th_sync should accumulate values with '|=' to be not dropped
  occasionally to the async state from sync
- remove duplicated code
- remove ASSERTION in OSD_EXEC_OP as it was previously done but
  returned again from orion by mistake

Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Change-Id: I6ee7eb8a086eae6e0a96364a90a192e6ac95903b
Reviewed-on: http://review.whamcloud.com/2530
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>