cluster/afr: Handle NULL fdctx in fsync

Problem:
If fdctx is NULL in afr_fsync, process crashes because
of NULL dereference.

Fix:
if fdctx is NULL, always say witnessed unstable write so
that fsyncs are done properly. Handled fdctx being null
in afr_delayed_changelog_post_op otherwise fsync stub is
never resumed and the mount was hanging.

Change-Id: Icacc900e9be63c29db3325cb0e19cc250adebaac
BUG: 978794
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/5258
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>