cluster/afr: Prevent double big lock when data self-heal loops are not spawned

The steps in normal data self heal:
1) take big lock by self-heal frame. Get the xattrs/stat to decide
source, sink information.
2) spawn loop frames which perform self-heal by taking small locks on
the file. Every time a new lock is taken and the old lock is released.
3) Before releasing the final small lock a big lock is taken by the
self-heal frame, and unlock on small-lock. Erasing of the pending xattrs
happen then the big unlock happen and that is the end of the data self-heal.

When a data self-heal is needed for a file and the fop
that triggers the self-heal is open with O_TRUNC. Fuse sends open then
an explicit truncate for this. Open triggers the self-heal but by the
time it tries to spawn the loops the file size is truncated to 0, so
no loops are formed.
These are the steps:
1) Take big lock by self-heal frame. Get the xattrs/stat to decide
source, sink information.
2) loop frames are not spawned. The big lock is not released.
3) One more big lock is taken by the same self-heal frame, Erasing of
the pending xattrs etc happen, now it does two big unlocks, but after
the first unlock, the information on which the locks were performed is
forgotten, so the next unlock becomes a no-op. So there is a stale big
lock on that file preventing further writes.

As a fix, if the loops are not spawned, use the previous big lock to
perform the rest of the operations needed in completing the data
self-heal. No need to have one more big lock.

Change-Id: Id03171269594e447b2b6d1331e362d83bd1e3430
BUG: 3506
Reviewed-on: http://review.gluster.com/339
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@gluster.com>

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