Don't abort starting the array if kernel does not support ppl

Don't abort starting the array if kernel does not support ppl

Change the behavior of assemble and create for consistency-policy=ppl
for external metadata arrays. If the kernel does not support ppl, don't
abort but print a warning and start the array without ppl
(consistency-policy=resync). No change for native md arrays because the
kernel will not allow starting the array if it finds an unsupported
feature bit in the superblock.

In sysfs_add_disk() check consistency_policy in the mdinfo structure
that represents the array, not the disk and read the current consistency
policy from sysfs in mdmon's manage_member(). This is necessary to make
sysfs_add_disk() honor the actual consistency policy and not what is in
the metadata. Also remove all the places where consistency_policy is set
for a disk's mdinfo - it is a property of the array, not the disk.

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