bioops.io_start() was being called in a situation where the buffer could

bioops.io_start() was being called in a situation where the buffer could
be brelse()'d afterwords instead of I/O being initiated.  When this occurs,
the buffer may contain softupdates-modified data which is never reverted,
resulting in serious filesystem corruption.  When io_start is called on a
buffer, I/O MUST be initiated and terminated with a biodone() or the buffer's
data may not be properly reverted.

Solve the problem by moving the io_start() call a little further on in the
code, after the potential brelse().

There is a possibility that this bug is responsible for the 'dirbad' panics
often reported in DragonFly and FreeBSD circles.