ceph: streamline request head structures in MDS client

The existence of the ceph_mds_request_head_old structure in the MDS
client code is no longer required due to improvements in handling
different MDS request header versions. This patch removes the now
redundant ceph_mds_request_head_old structure and replaces its usage
with the flexible and extensible ceph_mds_request_head structure.

Changes include:
- Modification of find_legacy_request_head to directly cast the
pointer to ceph_mds_request_head_legacy without going through the
old structure.
- Update sizeof calculations in create_request_message to use
offsetofend for consistency and future-proofing, rather than
referencing the old structure.
- Use of the structured ceph_mds_request_head directly instead of the
old one.

Additionally, this consolidation normalizes the handling of
request_head_version v1 to align with versions v2 and v3, leading to
a more consistent and maintainable codebase.

These changes simplify the codebase and reduce potential confusion
stemming from the existence of an obsolete structure.

Signed-off-by: Liang Jie <[e-mail]>
Reviewed-by: Viacheslav Dubeyko <[e-mail]>
Signed-off-by: Ilya Dryomov <[e-mail]>

fs/ceph/mds_client.c

···
 find_legacy_request_head(void *p, u64 features)
 {
 	bool legacy = !(features & CEPH_FEATURE_FS_BTIME);
-	struct ceph_mds_request_head_old *ohead;
+	struct ceph_mds_request_head *head;
 
 	if (legacy)
 		return (struct ceph_mds_request_head_legacy *)p;
-	ohead = (struct ceph_mds_request_head_old *)p;
-	return (struct ceph_mds_request_head_legacy *)&ohead->oldest_client_tid;
+	head = (struct ceph_mds_request_head *)p;
+	return (struct ceph_mds_request_head_legacy *)&head->oldest_client_tid;
 }
 
 /*
···
 	if (legacy)
 		len = sizeof(struct ceph_mds_request_head_legacy);
 	else if (request_head_version == 1)
-		len = sizeof(struct ceph_mds_request_head_old);
+		len = offsetofend(struct ceph_mds_request_head, args);
 	else if (request_head_version == 2)
 		len = offsetofend(struct ceph_mds_request_head, ext_num_fwd);
 	else
···
 		msg->hdr.version = cpu_to_le16(3);
 		p = msg->front.iov_base + sizeof(*lhead);
 	} else if (request_head_version == 1) {
-		struct ceph_mds_request_head_old *ohead = msg->front.iov_base;
+		struct ceph_mds_request_head *nhead = msg->front.iov_base;
 
 		msg->hdr.version = cpu_to_le16(4);
-		ohead->version = cpu_to_le16(1);
-		p = msg->front.iov_base + sizeof(*ohead);
+		nhead->version = cpu_to_le16(1);
+		p = msg->front.iov_base + offsetofend(struct ceph_mds_request_head, args);
 	} else if (request_head_version == 2) {
 		struct ceph_mds_request_head *nhead = msg->front.iov_base;
 
···
 	 * so we limit to retry at most 256 times.
 	 */
 	if (req->r_attempts) {
-	       old_max_retry = sizeof_field(struct ceph_mds_request_head_old,
+	       old_max_retry = sizeof_field(struct ceph_mds_request_head,
 					    num_retry);
 	       old_max_retry = 1 << (old_max_retry * BITS_PER_BYTE);
 	       if ((old_version && req->r_attempts >= old_max_retry) ||

include/linux/ceph/ceph_fs.h

···
 
 #define CEPH_MDS_REQUEST_HEAD_VERSION  3
 
-struct ceph_mds_request_head_old {
-	__le16 version;                /* struct version */
-	__le64 oldest_client_tid;
-	__le32 mdsmap_epoch;           /* on client */
-	__le32 flags;                  /* CEPH_MDS_FLAG_* */
-	__u8 num_retry, num_fwd;       /* count retry, fwd attempts */
-	__le16 num_releases;           /* # include cap/lease release records */
-	__le32 op;                     /* mds op code */
-	__le32 caller_uid, caller_gid;
-	__le64 ino;                    /* use this ino for openc, mkdir, mknod,
-					  etc. (if replaying) */
-	union ceph_mds_request_args_ext args;
-} __attribute__ ((packed));
-
 struct ceph_mds_request_head {
 	__le16 version;                /* struct version */
 	__le64 oldest_client_tid;