dmsetup: Support remove --deferred.

This patch adds a new flag --deferred to dmsetup remove. If this flag is
specified and the device is open, it is scheduled to be deleted on
close.

struct dm_info is extended.

The existing dm_task_get_info() is converted into a wrapper around the
new version dm_task_get_info_with_deferred_remove() so existing binaries
can still use the old smaller structure.

Recompiled code will pick up the new larger structure.

From: Mikulas Patocka <mpatocka@redhat.com>
RAID: Fail RAID4/5/6 creation if PE size is less than STRIPE_SIZE_MIN

The maximum stripe size is equal to the volume group PE size.  If that
size falls below the STRIPE_SIZE_MIN, the creation of RAID 4/5/6 volumes
becomes impossible.  (The kernel will fail to load a RAID 4/5/6 mapping
table with a stripe size less than STRIPE_SIZE_MIN.)  So, we report an
error if it is attempted.

This is very rare because reducing the PE size down that far limits the
size of the PV below that of modern devices.