HAMMER VFS - frontload kmalloc()'s when rebalancing

HAMMER VFS - frontload kmalloc()'s when rebalancing

* The rebalancing code must allocate upwards of 16MB of memory to hold
  copies of B-Tree nodes (~64*64*4K).  This is enough to blow out the
  emergency memory reserve used by the pageout daemon and deadlock the
  system in low memory situations.

* Refactor the allocations.  Allocate all the memory up-front so no
  major allocations occur while nodes in the B-Tree are held locked.

* There are probably other cases where this may become a problem.  With
  UFS it wasn't an issue because flushing a file was fairly unsophisticated.
  But with HAMMER certain aspects of the flush require B-Tree lookups and
  can't be dumbed down to a simple raw disk write.

  The rebalancing code was the most aggregious abuser of kernel memory
  though and that should now be fixed.

Reported-by: Francois Tigeot <ftigeot@wolfpond.org>