afr: treat appending writes as stable writes.

Durability of appending writes is implicit in the file size. Therefore
performing an explicit fsync() is unnecessary in such cases as self-heal
can check for the size of file when pending changelog is not unambiguous.

Change-Id: I05446180a91d20e0dbee5de5a7085b87d57f178a
BUG: 927146
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5501
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>