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 avalable.
