IB/hfi1: Add a safe wrapper for _rcd_get_by_index

hfi1_rcd_get_by_index assumes that the given index is in the correct
range.  In most cases this is correct because the index is bounded by
a loop.  For these cases, adding a range check to the function is
redundant.

For the use case that is not bounded by the loop range, a _safe wrapper
function is needed to validate the index before accessing the rcd array.

Add a _safe wrapper to _get_by_index to validate the index range.

Update appropriate call sites with the new _safe function.

Reviewed-by: Ira Weiny <ira.weiny@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>