[btrfs] fix check_direct_IO() for non-iovec iterators

looking for duplicate ->iov_base makes sense only for
iovec-backed iterators; for kvec-backed ones it's pointless,
for bvec-backed ones it's pointless and broken on 32bit (we
walk through an array of struct bio_vec accessing them as if
they were struct iovec; works by accident on 64bit, but on
32bit it'll blow up) and for pipe-backed ones it's pointless
and ends up oopsing.

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

fs/btrfs/inode.c

@@ -8618,7 +8618,7 @@ static ssize_t check_direct_IO(struct btrfs_root *root, struct kiocb *iocb,
 		goto out;
 
 	/* If this is a write we don't need to check anymore */
-	if (iov_iter_rw(iter) == WRITE)
+	if (iov_iter_rw(iter) != READ || !iter_is_iovec(iter))
 		return 0;
 	/*
 	 * Check to make sure we don't have duplicate iov_base's in this