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: NBaokun Li <libaokun1@huawei.com>
Reviewed-by: NAurelien Aptel <aaptel@suse.com>
Reviewed-by: NPaulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: NSteve French <stfrench@microsoft.com>

fs/cifs/cifs_dfs_ref.c

@@ -125,7 +125,7 @@ cifs_build_devname(char *nodename, const char *prepath)
  * @sb_mountdata:	parent/root DFS mount options (template)
  * @fullpath:		full path in UNC format
  * @ref:		optional server's referral
- *
+ * @devname:		return the built cifs device name if passed pointer not NULL
  * creates mount options for submount based on template options sb_mountdata
  * and replacing unc,ip,prefixpath options with ones we've got form ref_unc.
  *