Handle EXPORT_OPTION_COMMIT if FSAL_O_SYNC is not honored

FSAL_O_SYNC is not always honored, so just setting FSAL_O_SYNC has no
guaranty that this write will be a stable write. Instead, we pretend
that the caller asked for the stable write!

Signed-off-by: Malahal Naineni <malahal@us.ibm.com>
Acked-by: Marc Eshel <eshel@us.ibm.com>