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/file.c

@@ -715,7 +715,7 @@ static void ceph_aio_retry_work(struct work_struct *work)
 			CEPH_OSD_FLAG_ONDISK |
 			CEPH_OSD_FLAG_WRITE;
 	req->r_base_oloc = orig_req->r_base_oloc;
-	req->r_base_oid = orig_req->r_base_oid;
+	ceph_oid_copy(&req->r_base_oid, &orig_req->r_base_oid);
 
 	ret = ceph_osdc_alloc_messages(req, GFP_NOFS);
 	if (ret) {