fs/romfs/inode.c: trivial improvements

- There are no lists in fs/romfs/inode.c, so using list_entry
  is a bit confusing.  Replace it with container_of.

- It is unnecessary to cast the return value of
  kmem_cache_alloc, since it returns a void* pointer.

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>