disksort read/write order

[utzoo!henry]

In /usr/sys/dev/dsort.c, the general seek-sort routine used by hp.c
et al, the code as distributed queues reads before writes.  This is
WRONG, the intent was to put writes before reads (I asked Dennis Ritchie
at the San Francisco Usenix meeting).  The result is that on a very busy
system, it can take a looooong time for data to get out to disk, which
is risky and can hurt performance.

The fix is simple;  in the two if's that check B_READ flags, "ap" should
be changed to "bp" and vice-versa.

This bug has been known for some time, but hasn't been seen on the net
yet.  It was discovered by Peter Hardie at University of Saskatchewan.