hypfs: Add clarification for "weight_min" attribute

The "weight_min" attribute got the wrong name. The value represents
the number of non-stopped (operating) CPUS. Therefore add a note and
rename the struct member to "ocpus".

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>