x86: fix unusable squashfs images by adding missing padding

x86: fix unusable squashfs images by adding missing padding

It was reported, that after image generation rework
x86-64-generic-squashfs-rootfs.img image won't boot on XenServer x86_64
anymore:

 F2FS-fs (xvda): Magic Mismatch, valid(0xf2f52010) - read(0x84289960)
 F2FS-fs (xvda): Can't find valid F2FS filesystem in 1th superblock
 F2FS-fs (xvda): Magic Mismatch, valid(0xf2f52010) - read(0x4e8ee223)
 F2FS-fs (xvda): Can't find valid F2FS filesystem in 2th superblock
 List of all partitions:
 ca00            4207 xvda
  driver: vbd
 No filesystem could mount root, tried:
  ext3
  ext2
  ext4
  squashfs
  iso9660
  f2fs

 Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(202,0)

So lets fix this by adding back padding which was introduced in commit
a17d9482f5e2 ("x86: image: fix small disk space in squashfs overlay").

Ref: FS#3036
Fixes: 258f070d1a4f ("x86: fix missing squashfs and ext4 rootfs images")
Fixes: cb007a7bf619 ("x86: switch image generation to new code")
Signed-off-by: Petr Štetiar <ynezz@true.cz>