drmgr: Don't return unsuable CPU from get_available_cpu()
drmgr: Don't return unsuable CPU from get_available_cpu()

From: Bharata B Rao <37852461ec08f94420c663824224090aed552843@linux.vnet.ibm.com>

When querying CPU by name or index (and not count), let
get_available_cpu() return NULL for CPU that is marked as unusable.
This will prevent the callers (eg. add_cpus()) from infinitely
querying for the same CPU.

Signed-off-by: Bharata B Rao <37852461ec08f94420c663824224090aed552843@linux.vnet.ibm.com>

shenki/powerpc-utils,shenki/powerpc-utils,shenki/powerpc-utils