imsm: make messages more understandable

imsm: make messages more understandable

The printed messages should be more appropriate and understandable
for user. If maxsize is equal 0, this means there is no free space left
on device. If size is greater than maxsize, this means there is not enough
space to create a new volume of given size.

Acked-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Marcin Labun <marcin.labun@intel.com>
Signed-off-by: Lukasz Dorau <lukasz.dorau@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>