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 via the external 'dmsetup' executable.  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>