fs: 9p: fix v9fs_file_open writeback fid error check

IS_ERR() and PTR_ERR() use wrong pointer, it should be
writeback_fid, fix it.

Link: http://lkml.kernel.org/r/20210330130632.1054357-1-yangyingliang@huawei.com
Fixes: 5bfe97d7

 ("9p: Fix writeback fid incorrectly being attached to dentry")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
[Dominique: adjusted commit summary]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>