Kernel support for HAMMER:

Kernel support for HAMMER:

* Add another type to the bio->bio_caller_info1 union

* Add two new flags to getblk(), used by the cluster code.

  GETBLK_SZMATCH	- Tell getblk() to fail and return NULL if a
			  pre-existing buffer's size does not match
			  the requested size (this prevents getblk()
			  from doing a potentially undesired bwrite()
			  sequence).

  GETBLK_NOWAIT		- Tell getblk() to use a non-blocking lock.

* pop_bio() now returns the previous BIO (or NULL if there is no previous
  BIO).  This allows HAMMER to chain bio_done()'s

* Fix a bug in cluster_read().  The cluster code's read-ahead at the
  end could go past the caller-specified limit and force a block to
  the wrong block size.