xfs: also flush the RT device cache in xlog_write_iclog

xfs: also flush the RT device cache in xlog_write_iclog

The cache flush before writing the CIL start record no only needs to
ensure any metadata covered by the overwritten part of the log is on
stable storage, but also that any data pointed to by metadata logged
is on stable storage, as otherwise log recovery could created allocated
blocks that point to stale data.  Fortunately the code already
handles this right for the data device, but it also needs to flush
the RT device for this to work for data on the RT device.

Also update the comments to explicitly mention this case.

This omission goes back to the first days of cache control in XFS.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>