server: Move ->size from mmd to struct vss_task.

server: Move ->size from mmd to struct vss_task.

This field stores the size of the memory mapping of the current audio
file, if any. No command handler ever changes the value, and the
value is not read from com_stat(), so there is no reason for ->size
to be part of the mmd shared memory area.

Unlike ->size, the pointer to the mapping is stored in struct vss_task.
Given that vss.c is the only file which reads ->map or ->size, it
makes sense to store both values together in the vss task struct.