libfdisk: use BLKPG ioctls to inform kernel about changes

libfdisk: use BLKPG ioctls to inform kernel about changes

This patch introduces fdisk_reread_changes(). The function is
less invasive alternative to fdisk_reread_partition_table().

The new function uses BLKPG ioctls for modified partitions. The
another partitions are not affected. This solution allows to
successfully use fdisks on disk where some partitions are still use
(mounted). For example if you want to resize the last partition on the
device.

Signed-off-by: Karel Zak <kzak@redhat.com>