Restore previous ordering of BUFFER_FLUSH_START probe. I had wanted to

Restore previous ordering of BUFFER_FLUSH_START probe.  I had wanted to
make it include the time for the possible smgropen() call, but that
results in a null pointer dereference :-(.

An alternative solution would be to fetch the buffer tag instead of
looking at *reln, but I'll just put it back as it was for the moment.

BTW, this indicates that DTrace probes evaluate their arguments even
when nominally inactive.  What was that about "zero cost", again?