storage/posix: honor O_SYNC and O_DSYNC sent in @flags of writev()

Historic bug - posix_writev() has been inspecting pfd->flushwrites for
performing fsync() after write, instead of @flags for O_SYNC|O_DSYNC.

pfd->flushwrites was never set anywhere and is unused completely. This
is behavior from the time before anonymous FD where open() had @wbflags
param. This is a leftover from that cleanup.

Change-Id: Id9bfe562a60db4eb3bd0a7705bdba91f2df2f3ec
BUG: 916372
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4738
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>