[stats] allow knet_link_get_status to operate in readlock context

• add per link stats mutex
• use per link stats mutex across the board

note: some threads need to lock for a slightly longer period of time than
strictly necessary to avoid reverse-order locking with other mutexes.

Signed-off-by: Christine Caulfield <ccaulfie@redhat.com>
Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>