efi_loader: correct EFI_BLOCK_IO_PROTOCOL.Media.LastBlock

The field Media.LastBlock of the EFI_BLOCK_IO_PROTOCOL.Media was filled
incorrectly both for block devices as well as for partitions.

The field must be filled with the index of the last logical block (LBA):

* block devices: device size minus 1
* partitions: partition size minus 1

Reported-by: Alexander von Gluck IV <kallisti5@unixzen.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>