syncenv: be robust against spurious wake()s

In the current implementation, when the callers of synctasks perform
a spurious wake() of a sleeping synctask (i.e, an extra wake() soon
after a wake() which already woke up a yielded synctask), there is
now a possibility of two sync threacs picking up the same synctask.
This can result in a crash. The fix is to change ->slept = 0|1 and
membership of synctask in runqueue atomically.

Today we dequeue a task from the runqueue in syncenv_task(), but
reset ->slept = 0 much later in synctask_switchto() in an unlocked
manner -- which is safe, when there are no spurious wake()s.

However, this opens a race window where, if a second wake() happens
after the dequeue, but before setting ->slept = 0, it results in
queueing the same synctask in the runqueue once again, and get
picked up by a different synctask.

This is has been diagnosed to be the crashes in the regression tests
of http://review.gluster.org/4784. However that patch still has a
spurious wake() [the trigger for this bug] which is yet to be fixed.

BUG: 948686
Change-Id: I51858e887cad2680e46fb973629f8465f4429363
Original-author: Anand Avati <avati@redhat.com>
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/4833
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>