Overflowing refreservation is bad

Someone came to me and pointed out that you could pretty
readily cause the refreservation calculation to exceed
264, given the 217 multiplier in it, and produce
refreservations wildly less than the actual volsize in cases where
it should have failed.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #15996