direct_IO: use iov_iter_rw() instead of rw everywhere

The rw parameter to direct_IO is redundant with iov_iter->type, and
treated slightly differently just about everywhere it's used: some users
do rw & WRITE, and others do rw == WRITE where they should be doing a
bitwise check. Simplify this with the new iov_iter_rw() helper, which
always returns either READ or WRITE.

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

fs/xfs/xfs_aops.c

@@ -1503,7 +1503,7 @@ xfs_vm_direct_IO(
 	struct inode		*inode = iocb->ki_filp->f_mapping->host;
 	struct block_device	*bdev = xfs_find_bdev_for_inode(inode);
 
-	if (rw & WRITE) {
+	if (iov_iter_rw(iter) == WRITE) {
 		return __blockdev_direct_IO(iocb, inode, bdev, iter, offset,
 					    xfs_get_blocks_direct,
 					    xfs_end_io_direct_write, NULL,