zfs: Fix build on 32-bit platforms after most recent import.

unsigned long is not a uint64_t on 32-bit platforms. The zfs.4
manpage documents this variable as a uint, and it is only compared
with other variables of type int, so uint_t makes more sense than
unsigned long.

(I also wasn't sure if ULONG would work as a ZFS_MODULE_PARAM type
on other OS's)