The rcd array can be accessed from user context or during interrupts.
Protecting this with a mutex isn't a good idea because the mutex should
not be used from an IRQ.

Protect the allocation and freeing of rcd array elements with a
spinlock.
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>