This patch fixes three problems:

1. Fix NFS ability to grow a file using the truncate call.
   This problem usually appears as a kernel memory fault with the
   panicing thread having the following stack trace segment:

   [...]
   5 _OtsZero
   6 fs_setattr
   [...]

2. Fix I/O errors that can occur when writing to large AdvFS files.
   This problem can also cause a kernel memory fault panic in
   imm_remove_page_map().

3. Fix problems which can occur when an AdvFS domain is near full, and
   there is much activity adding and deleting data to files in that domain.
   Possible problems that can occur are:

      (a) data inconsistency in the files having data added or deleted
      (b) domain panic with the string, "dealloc_bits_page: can't clear
          a bit twice!"
      (c) system panic with the string, "block offset not aligned on
          cluster boundary".


