sctp: better integer overflow check in sctp_auth_create_key()

The check from commit 30c2235c is incomplete and cannot prevent
cases like key_len = 0x80000000 (INT_MAX + 1).  In that case, the
left-hand side of the check (INT_MAX - key_len), which is unsigned,
becomes 0xffffffff (UINT_MAX) and bypasses the check.

However this shouldn't be a security issue.  The function is called
from the following two code paths:

 1) setsockopt()

 2) sctp_auth_asoc_set_secret()

In case (1), sca_keylength is never going to exceed 65535 since it's
bounded by a u16 from the user API.  As such, the key length will
never overflow.

In case (2), sca_keylength is computed based on the user key (1 short)
and 2 * key_vector (3 shorts) for a total of 7 * USHRT_MAX, which still
will not overflow.

In other words, this overflow check is not really necessary.  Just
make it more correct.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Cc: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>