new helper: iov_iter_rw()

Get either READ or WRITE out of iter->type.
Signed-off-by: Omar Sandoval <osandov@osandov.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/uio.h

@@ -111,6 +111,14 @@ static inline bool iter_is_iovec(struct iov_iter *i)
 	return !(i->type & (ITER_BVEC | ITER_KVEC));
 }
 
+/*
+ * Get one of READ or WRITE out of iter->type without any other flags OR'd in
+ * with it.
+ *
+ * The ?: is just for type safety.
+ */
+#define iov_iter_rw(i) ((0 ? (struct iov_iter *)0 : (i))->type & RW_MASK)
+
 /*
  * Cap the iov_iter by given limit; note that the second argument is
  * *not* the new size - it's upper limit for such.  Passing it a value