af_key: Fix send_acquire race with pfkey_register

The function pfkey_send_acquire may race with pfkey_register
(which could even be in a different name space).  This may result
in a buffer overrun.

Allocating the maximum amount of memory that could be used prevents
this.

Reported-by:  <syzbot+1e9af9185d8850e2c2fa@syzkaller.appspotmail.com>
Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>