get rid of separate multipage fault-in primitives

* the only remaining callers of "short" fault-ins are just as happy with generic
variants (both in lib/iov_iter.c); switch them to multipage variants, kill the
"short" ones
* rename the multipage variants to now available plain ones.
* get rid of compat macro defining iov_iter_fault_in_multipage_readable by
expanding it in its only user.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ntfs/file.c

@@ -1850,7 +1850,7 @@ again:
 		 * pages being swapped out between us bringing them into memory
 		 * and doing the actual copying.
 		 */
-		if (unlikely(iov_iter_fault_in_multipages_readable(i, bytes))) {
+		if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
 			status = -EFAULT;
 			break;
 		}