Revert "ipv6/sit: use DEV_STATS_INC() to avoid data-races"

This reverts commit 222cc043 which is
commit cb34b7cf upstream.

The DEV_STATS_INC() macro is going away in a follow-on fix as
implementing it broke the abi.  If this is needed, please add it back
after DEV_STATS_INC() is added to the tree.

Bug: 161946584
Change-Id: I1c7cb7a20eaf4db4fef47af3cccd37d80168f5b3
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>