libfrog: always fsync when flushing a device

Always call fsync() when we're flushing a device, even if it is a block
device.  It's probably redundant to call fsync /and/ BLKFLSBUF, but the
latter has odd behavior so we want to make sure the standard flush
methods have a chance to run first.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>