sctp: integer overflow in sctp_auth_create_key()

The previous commit 30c2235c is incomplete and cannot prevent integer
overflows. For example, when key_len is 0x80000000 (INT_MAX + 1), the
left-hand side of the check, (INT_MAX - key_len), which is unsigned,
becomes 0xffffffff (UINT_MAX) and bypasses the check.
Signed-off-by: NXi Wang <xi.wang@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>