bawrite(). it's logically more correct (doesn't return an error code,
because it's async; bdwrite is also async), it still writes things
in-order, it makes sure the proper accountins is done (see the
wasdelayed cases in bwrite()), and it allows writes to vnodes on volumes
mountd with the MNT_ASYNC to be converted into delayed writes the way
God, err, Kirk intended. Convert synchronous bwrite()s on MNT_ASYNC
file systems to delayed writes.
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