libceph: variable-sized ceph_object_id

Currently ceph_object_id can hold object names of up to 100
(CEPH_MAX_OID_NAME_LEN) characters.  This is enough for all use cases,
expect one - long rbd image names:

- a format 1 header is named "<imgname>.rbd"
- an object that points to a format 2 header is named "rbd_id.<imgname>"

We operate on these potentially long-named objects during rbd map, and,
for format 1 images, during header refresh.  (A format 2 header name is
a small system-generated string.)

Lift this 100 character limit by making ceph_object_id be able to point
to an externally-allocated string.  Apart from being able to work with
almost arbitrarily-long named objects, this allows us to reduce the
size of ceph_object_id from >100 bytes to 64 bytes.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/ioctl.c

@@ -213,7 +213,7 @@ static long ceph_ioctl_get_dataloc(struct file *file, void __user *arg)
 		 ceph_ino(inode), dl.object_no);
 
 	oloc.pool = ceph_file_layout_pg_pool(ci->i_layout);
-	ceph_oid_set_name(&oid, dl.object_name);
+	ceph_oid_printf(&oid, "%s", dl.object_name);
 
 	r = ceph_oloc_oid_to_pg(osdc->osdmap, &oloc, &oid, &pgid);
 	if (r < 0) {