per-filesystem syncer threads

[Venkatesh Srinivas]

Hi,

There is a kernel process, 'syncer', that periodically fsync()s dirty
vnodes on each filesystem and syncs each filesystem.

This patchset (http://leaf.dragonflybsd.org/~vsrinivas/syncer) {or
http://gitweb.dragonflybsd.org/~vsrinivas/vmflush if you prefer git
trees} allows filesystems to flag themselves with MNT_THR_SYNC; if
they are so flagged, their periodic fsync() and sync paths will be
called from a per-filesystem thread.

The patchset doesn't make any filesystems that way, but I've tested a
little bit with UFS and tmpfs.

Any comments/patch reviews/etc would be greatly appreciated!

Thanks,
-- vs;