RDMA/rxe: Fix race in do_task() when draining

When do_task() exhausts its iteration budget (!ret), it sets the state
to TASK_STATE_IDLE to reschedule, without a secondary check on the
current task->state. This can overwrite the TASK_STATE_DRAINING state
set by a concurrent call to rxe_cleanup_task() or rxe_disable_task().

While state changes are protected by a spinlock, both rxe_cleanup_task()
and rxe_disable_task() release the lock while waiting for the task to
finish draining in the while(!is_done(task)) loop. The race occurs if
do_task() hits its iteration limit and acquires the lock in this window.
The cleanup logic may then proceed while the task incorrectly
reschedules itself, leading to a potential use-after-free.

This bug was introduced during the migration from tasklets to workqueues,
where the special handling for the draining case was lost.

Fix this by restoring the original pre-migration behavior. If the state is
TASK_STATE_DRAINING when iterations are exhausted, set cont to 1 to
force a new loop iteration. This allows the task to finish its work, so
that a subsequent iteration can reach the switch statement and correctly
transition the state to TASK_STATE_DRAINED, stopping the task as intended.

Fixes: 9b4b7c1f9f54 ("RDMA/rxe: Add workqueue support for rxe tasks")
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
Link: https://patch.msgid.link/20250919025212.1682087-1-hanguidong02@gmail.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>

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