Re-enable IO_NOCACHE, and use is in vnd.

Ensure that io to a file backing a vnd is IO_SYNC, so IO to a
vnd device is both synchronous and not cached in the buffer cache.

This allows the "mount" regress to work repeatably, and avoids
a situation where when the buffer cache cleaner runs to clear
dirty buffers while people are waiting, it actually increases the
dirty buffers when the writes to the underlying vnd are also
delayed.

ok bluhm@

0 files changed, 0 insertions, 0 deletions