lib/iov_iter: initialize "flags" in new pipe_buffer

lib/iov_iter: initialize "flags" in new pipe_buffer

The functions copy_page_to_iter_pipe() and push_pipe() can both
allocate a new pipe_buffer, but the "flags" member initializer is
missing.

Fixes: 96607715cf28 ("new iov_iter flavour: pipe-backed")
To: Alexander Viro <viro@zeniv.linux.org.uk>
To: linux-fsdevel@vger.kernel.org
To: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>