hammer2 - Add vfs.hammer2.limit_dirty_inodes

hammer2 - Add vfs.hammer2.limit_dirty_inodes

* Add vfs.hammer2.limit_dirty_inodes which causes hammer2 to
  immediately begin fsync()ing dirty inodes when the count exceeds
  the limit.  Set a reasonable limit.

* Fixes issues on slower storage when the syncer is unable to
  keep up with the userland frontend, causing the number of
  dirty inodes to increase almost unbounded (limited only by
  kern.maxvnodes).

  Allowing a large number of dirty inodes to accumulate can
  can result in a situation where the clean device buffer
  underpinning the dirty inode is discarded by the kernel before
  the filesystem is able to flush it, forcing additional disk reads
  and slowing things down even more.

* Improve the operation of speedup_syncer() by limiting the
  rate at which we call the function.  It is now called a maximum
  of approximately once per tick (each call speeding up a sync
  by one second).