block: avoid infinite loop in get_task_io_context()

block: avoid infinite loop in get_task_io_context()

Calling get_task_io_context() on a exiting task which isn't %current can
loop forever. This triggers at boot time on my dev machine.

BUG: soft lockup - CPU#3 stuck for 22s ! [mountall.1603]

Fix this by making create_task_io_context() returns -EBUSY in this case
to break the loop.

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Alan Cox <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>