[PKT_SCHED] ematch: Fix build warning.

Commit 954415e3

 ("[PKT_SCHED] ematch:
tcf_em_destroy robustness") removed a cast on em->data when
passing it to kfree(), but em->data is an integer type that can
hold pointers as well as other values so the cast is necessary.

Signed-off-by: David S. Miller <davem@davemloft.net>