ext2: Fix some kernel-doc warnings

The current code provokes some kernel-doc warnings:
fs/ext2/dir.c:417: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

Signed-off-by: Bo Liu <liubo03@inspur.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jan Kara <jack@suse.cz>