netfilter: ebtables: reject bogus getopt len value

syzkaller reports splat:
------------[ cut here ]------------
Buffer overflow detected (80 < 137)!
Call Trace:
 do_ebt_get_ctl+0x2b4/0x790 net/bridge/netfilter/ebtables.c:2317
 nf_getsockopt+0x72/0xd0 net/netfilter/nf_sockopt.c:116
 ip_getsockopt net/ipv4/ip_sockglue.c:1778 [inline]

caused by a copy-to-user with a too-large "*len" value.
This adds a argument check on *len just like in the non-compat version
of the handler.

Before the "Fixes" commit, the reproducer fails with -EINVAL as
expected:
1. core calls the "compat" getsockopt version
2. compat getsockopt version detects the *len value is possibly
   in 64-bit layout (*len != compat_len)
3. compat getsockopt version delegates everything to native getsockopt
   version
4. native getsockopt rejects invalid *len

-> compat handler only sees len == sizeof(compat_struct) for GET_ENTRIES.

After the refactor, event sequence is:
1. getsockopt calls "compat" version (len != native_len)
2. compat version attempts to copy *len bytes, where *len is random
   value from userspace

Fixes: fc66de8e

 ("netfilter/ebtables: clean up compat {get, set}sockopt handling")
Reported-by:  <syzbot+5accb5c62faa1d346480@syzkaller.appspotmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>