wait: use LIST_HEAD_INIT() to initialize wait_queue_head

Replace the open-coded initialization with the right macro.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210601151120.329223-1-jwi@linux.ibm.com