keys, dns: Fix missing size check of V1 server-list header

The dns_resolver_preparse() function has a check on the size of the
payload for the basic header of the binary-style payload, but is missing
a check for the size of the V1 server-list payload header after
determining that's what we've been given.

Fix this by getting rid of the the pointer to the basic header and just
assuming that we have a V1 server-list payload and moving the V1 server
list pointer inside the if-statement.  Dealing with other types and
versions can be left for when such have been defined.

This can be tested by doing the following with KASAN enabled:

    echo -n -e '\x0\x0\x1\x2' | keyctl padd dns_resolver foo @p

and produces an oops like the following:

    BUG: KASAN: slab-out-of-bounds in dns_resolver_preparse+0xc9f/0xd60 net/dns_resolver/dns_key.c:127
    Read of size 1 at addr ffff888028894084 by task syz-executor265/5069
    ...
    Call Trace:
      dns_resolver_preparse+0xc9f/0xd60 net/dns_resolver/dns_key.c:127
     ...