UPSTREAM: net: arc_emac: fix sk_buff leak

UPSTREAM: net: arc_emac: fix sk_buff leak

EMAC could be disabled, while there is some sb_buff
in use. That buffers got lost for linux.

In order to reproduce run on device during active ethernet work:
    ifconfig eth0 down

Change-Id: I7bc8b140be0ea378bc63fd11c026dbaee3f4d042
Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
(cherry picked from git.kernel.org next/linux-next.git master
 commit b530b16413b7f99977ded50a3c4bebd2ea79c41b)