cluster/afr: Change order of unwind, resume for writev

Generally inode-write fops do transaction.unwind then
transaction.resume, but writev needs to make sure that
delayed post-op frame is placed in fdctx before unwind
happens. This prevents the race of flush doing the
changelog wakeup first in fuse thread and then this
writev placing its delayed post-op frame in fdctx.
This helps flush make sure all the delayed post-ops are
completed.

Change-Id: Ia78ca556f69cab3073c21172bb15f34ff8c3f4be
BUG: 888174
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4428
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>