Cast reservation flags before sending to MySQL.

Flags are stored in a smallint, which can only hold the first 16 bits
worth out of 32 bits of flags currently in use.

MySQL's overflow rules will treat any value > 0xffff as 0xffff, rather than
dropping the higher-order bits (flags), which means the stored value not
only loses the higher-order bits but corrupts the lower-order as well.

The 19.05 release extends the column to bigint (64 bit).

Bug 6969.