tmem: expose freeable memory

tmem: expose freeable memory

Expose tmem "freeable" memory for use by management tools.

Management tools looking for a machine with available
memory often look at free_memory to determine if there
is enough physical memory to house a new or migrating
guest.  Since tmem absorbs much or all free memory,
and since "ephemeral" tmem memory can be synchronously
freed, management tools need more data -- not only how
much memory is "free" but also how much memory is
"freeable" by tmem if tmem is told (via an already
existing tmem hypercall) to relinquish freeable memory.
This patch provides that extra piece of data (in MB).

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>