netfilter: Fix several warnings in compat_mtw_from_user().

Kill set but not used 'entry_offset'.

Add a default case to the switch statement so the compiler
can see that we always initialize off and size_kern before
using them.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>