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
      __key_create_or_update+0x453/0xdf0 security/keys/key.c:842
      key_create_or_update+0x42/0x50 security/keys/key.c:1007
      __do_sys_add_key+0x29c/0x450 security/keys/keyctl.c:134
      do_syscall_x64 arch/x86/entry/common.c:52 [inline]
      do_syscall_64+0x40/0x110 arch/x86/entry/common.c:83
      entry_SYSCALL_64_after_hwframe+0x62/0x6a

This patch was originally by Edward Adam Davis, but was modified by
Linus.

Fixes: b946001d3bb1 ("keys, dns: Allow key types (eg. DNS) to be reclaimed immediately on expiry")
Reported-and-tested-by:  <syzbot+94bbb75204a05da3d89f@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/0000000000009b39bc060c73e209@google.com/


Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: David Howells <dhowells@redhat.com>
Cc: Edward Adam Davis <eadavis@qq.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Jeffrey E Altman <jaltman@auristor.com>
Cc: Wang Lei <wang840925@gmail.com>
Cc: Jeff Layton <jlayton@redhat.com>
Cc: Steve French <sfrench@us.ibm.com>
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>