These tests are intended to make sure that NFS's use of the
b_{valid,dirty}{off,end} fields of struct buf is consistent with the
VM system's use of the underlying VM pages.

Test1:
	Open the file and write into the file, creating a buf
	with a valid range and a dirty range

	Fsync, flushing the dirty range

	Mmap and read the whole page.  Since only part of the page is
	valid, the VM system must re-read the invalid parts of the
	page.

Test2:
	This is the same as test1 without the fsync.  The VM system
	should first write out the dirty range and then read the rest
	of the page.  This is currently broken since the vnode_pager
	doesn't use the original buf for its i/o and therefore the
	information in b_dirtyoff, b_dirtyend is not available.
