Ensure space for EBR when resizing Extended partition (#664050)

When resizing an extended boot record we must ensure that at least 2
sectors is reserved in front of the nearest logical partition for the
Extended Boot Record.

Please note that unless specifically told otherwise, the Linux kernel
considers Extended Boot Records to be two sectors long, in order to
"leave room for LILO".

Closes Bug #664050 - Unable to resize extended partition