fs: decouple READ and WRITE from the block layer ops

Move READ and WRITE to kernel.h and don't define them in terms of block
layer ops; they are our generic data direction indicators these days
and have no more resemblance with the block layer ops.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/include/linux/uio.h b/include/linux/uio.h
index 6e22b54..d5aba15 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -125,7 +125,7 @@ static inline bool iter_is_iovec(const struct iov_iter *i)
  *
  * The ?: is just for type safety.
  */
-#define iov_iter_rw(i) ((0 ? (struct iov_iter *)0 : (i))->type & RW_MASK)
+#define iov_iter_rw(i) ((0 ? (struct iov_iter *)0 : (i))->type & (READ | WRITE))
 
 /*
  * Cap the iov_iter by given limit; note that the second argument is