xfs_io: add a new 'log_writes' command

xfs_io: add a new 'log_writes' command

Add a new 'log_writes' command to xfs_io so that we can add dm-log-writes
log marks.  It's helpful to allow users of xfs_io to adds these marks from
within xfs_io instead of waiting until after xfs_io exits because then they
are able to replay the dm-log-writes log up to immediately after another
xfs_io operation such as mwrite.  This isolates the log replay from other
operations that happen as part of xfs_io exiting (file handles being
closed, mmaps being torn down, etc.).  This also allows users to insert
multiple marks between different xfs_io commands.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Suggested-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>