block: online resize of disk partitions

Hi,

Following patch adds support for online resizing of a partition. This patch
is based on previously posted patches by Phillip Susi.

There are two patches. Out of which one is kernel patch and other one is
util-linux patch to add support of a user space utility "resizepart" to
allow resizing the partition.

This ioctl only resizes the partition size in kenrel and does not change
the size on disk. A user needs to make sure that corresponding changes
are made to disk data structures also using fdisk(or partx), if changes
are to be retained across reboot.

I made some changes according to the feedback received last time. Following
are changes since the version Phillip posted.

- RESIZE ioctl ignores the partition "start" and does not expect user to
  specify one. Caller needs to just specify "device", "partition number" and
  "size" of new partition. 

- Got rid of part_nr_sects_write_begin/part_nr_sects_write_end functions
  and replaced these with single part_nr_sects_write().

- Some sequence counter related changes are simply lifted from i_size_write().

- Initialized part->nr_sects_seq using seqcount_init().

Phillip, do let me know if I should put your signed-off-by also in the
patch.

Any feedback is welcome.

I did following test.

- Create a partition of 10MB on a disk using fdisk.
- Add this partition to a volume group
- Use fdisk to increase the partition size to 20MB. (First delete the
  partition and then create a new one of 20MB size).
- Use resizepart to extend partition size in kernel.
	resizepart /dev/sdc 1 40960
- Do pvresize on partition so that physical volume can be incrased in
  size online.
	pvresize /dev/sda1

pvresize does recognize the new size. Also lsblk and /proc/partitions
report the new size of partition.

Thanks
Vivek

[PATCH 1/2] block: add partition resize function to blkpg ioctl
[PATCH 2/2] resizepart: Utility to resize a partition
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