Description of problem:
The dm-stats facility provides a means to store a string as auxilliary data to a region that is set either when the region is created or via the @stats_set_aux method:

@stats_create - /1 dmstats this_is_aux_data
@stats_set_aux 1 this_is_aux_data_too

The userspace dmstats support will use this to track groups of regions defined by the user by storing a string description of the group members and other properties in the aux_data field.

Since users can create and delete groups at any time there needs to be a mechanism to remove a previously stored value or to set the field to the empty string.

This is not currently possible as the empty string is treated as whitespace leading to EINVAL from the message ioctl (as a workaround it is possible to set the field to a dummy character like '-' but this is not ideal).

Adding a new @stats_clear_aux message seems like the simplest change and should be a simple patch.

Version-Release number of selected component (if applicable):
kernel-3.10.0-*

How reproducible:
100%

Steps to Reproduce:
1. Create a region with aux_data
2. Attempt to re-set it to empty aux_data with @stats_set_aux


Actual results:
EINVAL with the empty string; at least one character must be set.

Expected results:
Ability to remove previously stored aux_data.

Additional info:
WIP groups support: https://git.fedorahosted.org/cgit/lvm2.git/log/?h=dev-bmr-dmstats-groups