imsm: PPL support

imsm: PPL support

Enable creating and assembling IMSM raid5 arrays with PPL. Update the
IMSM metadata format to include new fields used for PPL.

Add structures for PPL metadata. They are used also by super1 and shared
with the kernel, so put them in md_p.h.

Write the initial empty PPL header when creating an array. When
assembling an array with PPL, validate the PPL header and in case it is
not correct allow to overwrite it if --force was provided.

Write the PPL location and size for a device to the new rdev sysfs
attributes 'ppl_sector' and 'ppl_size'. Enable PPL in the kernel by
writing to 'consistency_policy' before the array is activated.

Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@gmail.com>