f2fs: add check for deleted inode

The syzbot reproducer mounts a f2fs image, then tries to unlink an
existing file. However, the unlinked file already has a link count of 0
when it is read for the first time in do_read_inode().

Add a check to sanity_check_inode() for i_nlink == 0.

[Chao Yu: rebase the code and fix orphan inode recovery issue]
Reported-by:  <syzbot+b01a36acd7007e273a83@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=b01a36acd7007e273a83


Fixes: 39a53e0c ("f2fs: add superblock and major in-memory structure")
Signed-off-by: Leo Stone <leocstone@gmail.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>