jffs2: remove from wait queue after schedule()

@wait is a local variable, so if we don't remove it from the wait queue
list, later wake_up() may end up accessing invalid memory.

This was spotted by eyes.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>