net: sched: initialize with 0 before setting erspan md->u

In fl_set_erspan_opt(), all bits of erspan md was set 1, as this
function is also used to set opt MASK. However, when setting for
md->u.index for opt VALUE, the rest bits of the union md->u will
be left 1. It would cause to fail the match of the whole md when
version is 1 and only index is set.

This patch is to fix by initializing with 0 before setting erspan
md->u.

Reported-by: Shuang Li <shuali@redhat.com>
Fixes: 79b1011c

 ("net: sched: allow flower to match erspan options")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>