From: Christoph Hellwig <hch@lst.de>

Change sysrq sync/remount from a magic bdflush hook to proper pdflush
operations.  The sync operation reuses most of the regular sys_sync path now
instead of implementing it's own superblock walking and (broken) local disk
detection, the remount implementation has been moved to super.c, cleaned up
and updated for the last two years locking changes.  It also shares some code
with the regular remount path now.