[EBTABLES]: Fix wraparounds in ebt_entries verification.

We need to verify that
    a) we are not too close to the end of buffer to dereference
    b) next entry we'll be checking won't be _before_ our

While we are at it, don't subtract unrelated pointers...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>