Sync filesystems before returning from MBR installation

We need to make sure the device/filesystem is synced, otherwise
syslinux installation might fail with:

| [...]
| Installing default MBR
| Installing syslinux as bootloader
| syslinux: /dev/sdb1: No such file or directory
| Fatal: Error executing syslinux (either try --fat16 or use grub?)

fsync-ing via dd isn't enough, so we need to make it
a full sync.

Closes: #779193