cifs: Fix separator when building path from dentry

Make sure to use the CIFS_DIR_SEP(cifs_sb) as path separator for
prefixpath too. Fixes a bug with smb1 UNIX extensions.

Fixes: a6b5058f ("fs/cifs: make share unaccessible at root level mountable")
Signed-off-by: Paulo Alcantara <palcantara@suse.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>

fs/cifs/dir.c

@@ -174,7 +174,7 @@ build_path_from_dentry_optional_prefix(struct dentry *direntry, bool prefix)
 
 		cifs_dbg(FYI, "using cifs_sb prepath <%s>\n", cifs_sb->prepath);
 		memcpy(full_path+dfsplen+1, cifs_sb->prepath, pplen-1);
-		full_path[dfsplen] = '\\';
+		full_path[dfsplen] = dirsep;
 		for (i = 0; i < pplen-1; i++)
 			if (full_path[dfsplen+1+i] == '/')
 				full_path[dfsplen+1+i] = CIFS_DIR_SEP(cifs_sb);