[XFS] Ensure a frozen filesystem has a clean log before writing the dummy

[XFS] Ensure a frozen filesystem has a clean log before writing the dummy
record.

The current Linux XFS freeze code is a mess. We flush the metadata buffers
out while we are still allowing new transactions to start and then fail to
flush the dirty buffers back out before writing the unmount and dummy
records to the log.

This leads to problems when the frozen filesystem is used for snapshots -
we do log recovery on a readonly image and often it appears that the log
image in the snapshot is not correct. Hence we end up with hangs, oops and
mount failures when trying to mount a snapshot image that has been created
when the filesystem has not been correctly frozen.

To fix this, we need to move th metadata flush to after we wait for all
current transactions to complete in teh second stage of the freeze. This
means that when we write the final log records, the log should be clean
and recovery should never occur on a snapshot image created from a frozen
filesystem.

SGI-PV: 959267
SGI-Modid: xfs-linux-melb:xfs-kern:28010a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>