part_efi: Fix partition size calculation due to inclusive ending LBA.

The ending LBA is inclusive. Hence, the partition size should be
((ending-LBA + 1) - starting-LBA) to get the proper partition size.

This is confirmed against the results from the parted tool.
(e.g. use parted /dev/sda -s unit S print) and observe the size.

Signed-off-by: Richard Retanubun <RichardRetanubun@RuggedCom.com>