glfs: remove O_SYNC
glfs: remove O_SYNC

O_SYNC makes IO a bit slow and as anyway we are ensuring the data
reaches onto nonvolatile disk with fdatasync() as part of flush op,
O_SYNC seems to be unworthy.

Signed-off-by: Prasanna Kumar Kalever <a78d8f98557446023f5b3ba9944d7be259a978a3@redhat.com>

open-iscsi/tcmu-runner,open-iscsi/tcmu-runner,agrover/tcmu-runner