fs: Convert kiocb rw_hint from enum to u16

In order to avoid kiocb bloat for per command iopriority support, rw_hint
is converted from enum to a u16. Added a guard around ki_hint assignment.

Signed-off-by: Adam Manzanares <adam.manzanares@wdc.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/aio.c

@@ -1434,7 +1434,7 @@ static int aio_prep_rw(struct kiocb *req, struct iocb *iocb)
 	req->ki_flags = iocb_flags(req->ki_filp);
 	if (iocb->aio_flags & IOCB_FLAG_RESFD)
 		req->ki_flags |= IOCB_EVENTFD;
-	req->ki_hint = file_write_hint(req->ki_filp);
+	req->ki_hint = ki_hint_validate(file_write_hint(req->ki_filp));
 	ret = kiocb_set_rw_flags(req, iocb->aio_rw_flags);
 	if (unlikely(ret))
 		fput(req->ki_filp);