Correct cases of mishandling of pending reads and writes to prevent

Correct cases of mishandling of pending reads and writes to prevent
them going negative - this consists of identifying a number of cases of
IO not going through the buffer cache and marking those buffers with
B_RAW - as well as fixing nfs_bio to show pending writes and reads through
the buffer cache via NFS

still has a problem with mishandling the counters I believe in the
async/sync fallback case where counters stay positive which will be
addressed seperately.

ok tedu@ deraadt@