libceph: rados pool namespace support

Add pool namesapce pointer to struct ceph_file_layout and struct
ceph_object_locator. Pool namespace is used by when mapping object
to PG, it's also used when composing OSD request.

The namespace pointer in struct ceph_file_layout is RCU protected.
So libceph can read namespace without taking lock.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
[idryomov@gmail.com: ceph_oloc_destroy(), misc minor changes]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

include/linux/ceph/ceph_fs.h

@@ -53,6 +53,7 @@ struct ceph_file_layout_legacy {
 	__le32 fl_pg_pool;      /* namespace, crush ruleset, rep level */
 } __attribute__ ((packed));
 
+struct ceph_string;
 /*
  * ceph_file_layout - describe data layout for a file/inode
  */
@@ -62,6 +63,7 @@ struct ceph_file_layout {
 	u32 stripe_count;  /* over this many objects */
 	u32 object_size;   /* until objects are this big */
 	s64 pool_id;        /* rados pool id */
+	struct ceph_string __rcu *pool_ns; /* rados pool namespace */
 };
 
 extern int ceph_file_layout_is_valid(const struct ceph_file_layout *layout);