Linux 4.9 compat: group_info changes

In Linux 4.9, torvalds/linux@81243ea, group_info changed from 2d array via
->blocks to 1d array via ->gid. We change the spl cred functions accordingly.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #581

module/spl/spl-cred.c

@@ -85,7 +85,9 @@ crgetngroups(const cred_t *cr)
 
 	gi = cr->group_info;
 	rc = gi->ngroups;
+#ifndef HAVE_GROUP_INFO_GID
 	/*
+	 * For Linux <= 4.8,
 	 * crgetgroups will only returns gi->blocks[0], which contains only
 	 * the first NGROUPS_PER_BLOCK groups.
 	 */
@@ -93,12 +95,16 @@ crgetngroups(const cred_t *cr)
 		WARN_ON_ONCE(1);
 		rc = NGROUPS_PER_BLOCK;
 	}
+#endif
 	return rc;
 }
 
 /*
  * Return an array of supplemental gids.  The returned address is safe
  * to use as long as the caller has taken a reference with crhold().
+ *
+ * Linux 4.9 API change, group_info changed from 2d array via ->blocks to 1d
+ * array via ->gid.
  */
 gid_t *
 crgetgroups(const cred_t *cr)
@@ -107,8 +113,12 @@ crgetgroups(const cred_t *cr)
 	gid_t *gids = NULL;
 
 	gi = cr->group_info;
+#ifdef HAVE_GROUP_INFO_GID
+	gids = KGIDP_TO_SGIDP(gi->gid);
+#else
 	if (gi->nblocks > 0)
 		gids = KGIDP_TO_SGIDP(gi->blocks[0]);
+#endif
 	return gids;
 }