parted: avoid unnecessary open/close on commit, and thus udev activity

parted: avoid unnecessary open/close on commit, and thus udev activity

* libparted/disk.c (ped_disk_commit): Open/close the underlying file
descriptor in this function, so that callees, ped_disk_commit_to_dev
and ped_disk_commit_to_os do not each perform open/close syscalls.
This saves an open/close pair, and thus avoids unneeded udev
activity on Linux.

Before this change, when calling commit() on a ped_disk, the
following would happen:

open /dev/sda
write partition table
close /dev/sda
open /dev/sda
ioctl (BLKRRPART)
close /dev/sda

This is rather inefficient, and causes 2 udev change events to be fired
for /dev/sda (+ the change events from the BLKRRPART), causing all kind
of scanning (blkid & friends) twice.

This patch fixes things to only open the device once.