coda: avoid flagging NULL inodes

coda: avoid flagging NULL inodes

Somehow we hit a negative dentry in coda_rename even after checking with
d_really_is_positive.  Maybe something raced and turned the new_dentry
negative while we were fixing up directory link counts.

Link: https://lkml.kernel.org/r/20210908140308.18491-5-jaharkes@cs.cmu.edu
Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>
Cc: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Jing Yangyang <jing.yangyang@zte.com.cn>
Cc: Xin Tan <tanxin.ctf@gmail.com>
Cc: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Cc: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>