p9_client_write(): avoid double p9_free_req()

Braino in "9p: switch p9_client_write() to passing it struct iov_iter *";
if response is impossible to parse and we discard the request, get the
out of the loop right there.

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