cluster/afr: Pre-op should be undone for non-piggyback post-op

Problem:
When fop fails post-op is always performed
over the network irrespective of whether pre-op is piggybacked
or not. Decrementing Pre-op-done count even for the piggybacked
ones is wrong.
I have added an assert for pre_op_done to be non-zero and when
dd of=a if=/dev/urandom bs=5M count=1000 is executed and a brick
is taken down, the mount is crashing.

Fix:
Decrement pre-op-done count only when the post-op is not
piggybacked.

Change-Id: Ie837251a43bfb437f0fada191302eeee60be1601
BUG: 863939
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4310
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>