fs: add kiocb alloc cache flag

If this kiocb can safely use the polled bio allocation cache, then this
flag must be set. Generally this can be set for polled IO, where we will
not see IRQ completions of the request.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/fs.h

@@ -319,6 +319,8 @@ enum rw_hint {
 /* iocb->ki_waitq is valid */
 #define IOCB_WAITQ		(1 << 19)
 #define IOCB_NOIO		(1 << 20)
+/* can use bio alloc cache */
+#define IOCB_ALLOC_CACHE	(1 << 21)
 
 struct kiocb {
 	struct file		*ki_filp;