ceph: switch TASK_INTERRUPTIBLE to TASK_KILLABLE

If the task is placed in the TASK_INTERRUPTIBLE state it will sleep
until either something explicitly wakes it up, or a non-masked signal
is received. Switch to TASK_KILLABLE to avoid the noises.

Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>