Avoid using BLKFLSBUF.

Avoid using BLKFLSBUF.

Now that we use O_DIRECT for all device IO, BLKFLSBUF is not needed to
ensure we get current data, and it can impose a cost if any flush-out
is needed.  So remove it.

To be safe, add O_DIRECT to one place where it isn't currently used:
when reading a bitmap.

Signed-off-by: NeilBrown <neilb@suse.de>