integer overflow on computation of refquota_slack

[ColinIanKing]

uint64_t refquota_slack = DMU_MAX_ACCESS * spa_asize_inflation;

Found by static analysis with CoverityScan 0.8.5

CID 150942 (#1 of 1): Unintentional integer overflow
(OVERFLOW_BEFORE_WIDEN)
overflow_before_widen: Potentially overflowing expression
67108864 * spa_asize_inflation with type int (32 bits, signed)
is evaluated using 32-bit arithmetic, and then used in a context
that expects an expression of type uint64_t (64 bits, unsigned).

DMU_MAX_ACCESS should be cast to a uint64_t otherwise the
multiplication of DMU_MAX_ACCESS with spa_asize_inflation will
be 32 bit and may lead to an overflow. Currently DMU_MAX_ACCESS
is 64 * 1024 * 1024, so spa_asize_inflation being 64 or more will
lead to an overflow.

[ColinIanKing]

I'm sending a patch to fix this issue

[ColinIanKing]

please pull fix from ColinIanKing@2b42d79