io-wq: add support for uncancellable work

Not all work can be cancelled, some of it we may need to guarantee
that it runs to completion. Allow the caller to set IO_WQ_WORK_NO_CANCEL
on work that must not be cancelled. Note that the caller work function
must also check for IO_WQ_WORK_NO_CANCEL on work that is marked
IO_WQ_WORK_CANCEL.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.h

@@ -12,6 +12,7 @@ enum {
 	IO_WQ_WORK_UNBOUND	= 32,
 	IO_WQ_WORK_INTERNAL	= 64,
 	IO_WQ_WORK_CB		= 128,
+	IO_WQ_WORK_NO_CANCEL	= 256,
 
 	IO_WQ_HASH_SHIFT	= 24,	/* upper 8 bits are used for hash key */
 };