md: Runtime support for multiple ppls

Increase PPL area to 1MB and use it as circular buffer to store PPL. The
entry with highest generation number is the latest one. If PPL to be
written is larger then space left in a buffer, rewind the buffer to the
start (don't wrap it).

Signed-off-by: Pawel Baldysiak <pawel.baldysiak@intel.com>
Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.h

@@ -236,6 +236,7 @@ enum mddev_flags {
 				 * never cause the array to become failed.
 				 */
 	MD_HAS_PPL,		/* The raid array has PPL feature set */
+	MD_HAS_MULTIPLE_PPLS,	/* The raid array has multiple PPLs feature set */
 };
 
 enum mddev_sb_flags {