arm64: Fix calculating kernel size for preload metadata

Cast &end to vm_offset_t before subtracting VM_MIN_KERNEL_ADDRESS to
ensure the resulting size is correct for PRELOAD_PUSH_VALUE.
Previously the correct size was effectively divided by sizeof(void *).

Reviewed by: andrew
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D53699