LU-12536 llite: release active extent on sync write commit

LU-12536 llite: release active extent on sync write commit

Processes can wait forever in osc_extent_wait() for the extent state
to change because the extent write is not started before the wait
begins. A 4.7 kernel change to generic_write_sync() modified it to
check IOCB_DSYNC instead of O_SYNC. Thus an active extent is not
released (written) in osc_io_commit_async() in the synchronous case.

Linux commit: dde0c2e79848298cc25621ad080d47f94dbd7cce

Cray-bug-id: LUS-7435
Signed-off-by: Ann Koehler <amk@cray.com>
Change-Id: I03fd3f17c0c21bcb176730a0edf449d2e5e0c108
Reviewed-on: https://review.whamcloud.com/35472
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>