libceph: fix non-default values check in apply_primary_affinity()

osd_primary_affinity array is indexed into incorrectly when checking
for non-default primary-affinity values.  This nullifies the impact of
the rest of the apply_primary_affinity() and results in misdirected
requests.

                if (osds[i] != CRUSH_ITEM_NONE &&
                    osdmap->osd_primary_affinity[i] !=
                                                ^^^
                                        CEPH_OSD_DEFAULT_PRIMARY_AFFINITY) {

For a pool with size 2, this always ends up checking osd0 and osd1
primary_affinity values, instead of the values that correspond to the
osds in question.  E.g., given a [2,3] up set and a [max,max,0,max]
primary affinity vector, requests are still sent to osd2, because both
osd0 and osd1 happen to have max primary_affinity values and therefore
we return from apply_primary_affinity() early on the premise that all
osds in the given set have max (default) values.  Fix it.

Fixes: http://tracker.ceph.com/issues/7954

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>