cifs: fix doc warnings in cifs_dfs_ref.c

Add description for `cifs_compose_mount_options` to fix the W=1 warnings:

 fs/cifs/cifs_dfs_ref.c:139: warning: Function parameter or
  member 'devname' not described in 'cifs_compose_mount_options'

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>