allocate 4096 bytes for 4k alligned devices

To make 4k scsi devices to work, allocated memory needs to be increased
accordingly

Tested in the following scenario:

modprobe scsi_debug dev_size_mb=100 sector_size=4096

parted /dev/sdc u b p
Model: Linux scsi_debug (scsi)
Disk /dev/sdc: 209715200B
Sector size (logical/physical): 4096B/4096B
Partition Table: msdos

Number  Start     End         Size        Type     File system  Flags
 1      262144B   2097151B    1835008B    primary               boot,
prep, type=41
 2      2097152B  200015871B  197918720B  primary               type=83

qemu-system-ppc64 -enable-kvm -M pseries -m 1024 -nographic -drive
file=/dev/sdc,if=none,format=raw,id=drive-scsi0 -device
scsi-block,drive=drive-scsi0,id=scsi0  -vga none

Signed-off-by: Dinar Valeev <dvaleev@suse.com>