ubifs: Fix wrong memory allocation

In create_default_filesystem() when we allocate the idx node we must use
the idx_node_size we calculated just one line before, not tmp, which
contains completely other data.

Fixes: c4de6d7e ("ubifs: Refactor create_default_filesystem()")
Cc: stable@vger.kernel.org # v4.20+
Reported-by: Naga Sureshkumar Relli <nagasure@xilinx.com>
Tested-by: Naga Sureshkumar Relli <nagasure@xilinx.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>