staging: lustre: osc: use rb_entry_safe

Use rb_entry_safe() instead of container_of() to simplify the code.
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/osc/osc_cache.c

@@ -142,10 +142,7 @@ static const char *oes_strings[] = {
 
 static inline struct osc_extent *rb_extent(struct rb_node *n)
 {
-	if (!n)
-		return NULL;
-
-	return container_of(n, struct osc_extent, oe_node);
+	return rb_entry_safe(n, struct osc_extent, oe_node);
 }
 
 static inline struct osc_extent *next_extent(struct osc_extent *ext)