kern/fs: Fix possible integer overflow in i386-pc mode with large partitions

kern/fs: Fix possible integer overflow in i386-pc mode with large partitions

The i386-pc mode supports MBR partition scheme where maximum partition
size is 2 TiB. In case of large partitions left shift expression with
unsigned long int "length" object may cause integer overflow making
calculated partition size less than true value. This issue is fixed by
increasing the size of "length" integer type.

Signed-off-by: Maxim Fomin <maxim@fomin.one>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>