cifs: use DFS root session instead of tcon ses

Use DFS root session whenever possible to get new DFS referrals
otherwise we might end up with an IPC tcon (tcon->ses->tcon_ipc) that
doesn't respond to them. It should be safe accessing
@ses->dfs_root_ses directly in cifs_inval_name_dfs_link_error() as it
has same lifetime as of @tcon.

Signed-off-by: Paulo Alcantara (SUSE) <[e-mail]>
Cc: [e-mail] # 6.2
Signed-off-by: Steve French <[e-mail]>

fs/cifs/misc.c

···
 		 * removing cached DFS targets that the client would eventually
 		 * need during failover.
 		 */
+		ses = CIFS_DFS_ROOT_SES(ses);
 		if (ses->server->ops->get_dfs_refer &&
 		    !ses->server->ops->get_dfs_refer(xid, ses, ref_path, &refs,
 						     &num_refs, cifs_sb->local_nls,