UML's block device driver does not support write barriers,
to support this this patch adds REQ_FLUSH suppport.
Every time the block layer sends a REQ_FLUSH we fsync() now
our backing file to guarantee data consistency.
Reported-and-tested-by: NRichard W.M. Jones <rjones@redhat.com>
Signed-off-by: NRichard Weinberger <richard@nod.at>