sd_fusing.py: Invoke ioctl to reread partitions directly

sd_fusing.py: Invoke ioctl to reread partitions directly

This commit drops blockdev(8) in favor for invoking ioctl(2) directly,
to be able to catch EBUSY errors that happen to occur due to not yet
known kernel/udev timinig issue.

This commit also drops sfdisk --no-reread flag, which affects issuing
ioctl to reread partition table _before_ actual repartitioning.  ioctl
to reread partition table after writing changes is invoked unconditionally
and can't be disabled.

Change-Id: I105c445f9fdb3d1d2eee7ffd2de3f7ad04920f13
Signed-off-by: Karol Lewandowski <k.lewandowsk@samsung.com>