fs/sysv: Null check to prevent null-ptr-deref bug

sb_getblk(inode->i_sb, parent) return a null ptr and taking lock on
that leads to the null-ptr-deref bug.

Reported-by:  <syzbot+aad58150cbc64ba41bdc@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=aad58150cbc64ba41bdc


Signed-off-by: Prince Kumar Maurya <princekumarmaurya06@gmail.com>
Message-Id: <20230531013141.19487-1-princekumarmaurya06@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>