vfs: add the RWF_HIPRI flag for preadv2/pwritev2

This adds a flag that tells the file system that this is a high priority
request for which it's worth to poll the hardware.  The flag is purely
advisory and can be ignored if not supported.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Stephen Bates <stephen.bates@pmcs.com>
Tested-by: Stephen Bates <stephen.bates@pmcs.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -320,6 +320,7 @@ struct writeback_control;
 #define IOCB_EVENTFD		(1 << 0)
 #define IOCB_APPEND		(1 << 1)
 #define IOCB_DIRECT		(1 << 2)
+#define IOCB_HIPRI		(1 << 3)
 
 struct kiocb {
 	struct file		*ki_filp;