Fix: populate possible cpus array len cache with fd tracker lock

Fix: populate possible cpus array len cache with fd tracker lock

get_possible_cpus_array_len internally opens and closes file
descriptors. Initialize the cache with fd tracker lock held to prevent
concurrent use of close/closeall/closefrom.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ib16c73f11af0b0cb163b38b0b096a86f6ae86325