barrier : fsync call over NFS should be barriered when barrier is enabled

barrier_fsync was not getting hit when fsync call is issued from the
application over NFS mount.
Investigation reveals that NFS doesn't send an explicit fsync call to the
glusterfs server, however NFS converts it with a stable write with O_DSYNC
flag, so for a fsync call over NFS mount point, barrier feature always
needs to check for writev and its correspoding flags. So to meet all the
coditions of write with O_SYNC and fsync getting blocked when barrier is
enabled the check has been modified with following:

      if (!((flags | fd->flags) & (O_SYNC | O_DSYNC)))

Point to be noted here is we need to check both fd's flag and the explicit
flag  as for NFS write fd->flags is 0.

Bug ID : 1091902
Change-Id: Ifd5377fca2cc93bb72158dd525b2aab8b4164ca8
Signed-off-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-on: http://review.gluster.org/7633
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>