super1: PPL support

super1: PPL support

Enable creating and assembling raid5 arrays with PPL for 1.x metadata.

When creating, reserve enough space for PPL and store its size and
location in the superblock and set MD_FEATURE_PPL bit. Write an initial
empty header in the PPL area on each device. PPL is stored in the
metadata region reserved for internal write-intent bitmap, so don't
allow using bitmap and PPL together.

While at it, fix two endianness issues in write_empty_r5l_meta_block()
and write_init_super1().

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