libceph: use pg_num_mask instead of pgp_num_mask for pg.seed calc

Fix a typo that used the wrong bitmask for the pg.seed calculation.  This
is normally unnoticed because in most cases pg_num == pgp_num.  It is, however,
a bug that is easily corrected.

CC: stable@vger.kernel.org
Signed-off-by: NSage Weil <sage@inktank.com>
Reviewed-by: NAlex Elder <alex.elder@linary.org>

net/ceph/osdmap.c

@@ -1129,7 +1129,7 @@ static int *calc_pg_raw(struct ceph_osdmap *osdmap, struct ceph_pg pgid,
 
 	/* pg_temp? */
 	pgid.seed = ceph_stable_mod(pgid.seed, pool->pg_num,
-				    pool->pgp_num_mask);
+				    pool->pg_num_mask);
 	pg = __lookup_pg_mapping(&osdmap->pg_temp, pgid);
 	if (pg) {
 		*num = pg->len;