fs: 9p: remove unneeded variable

Fix the following coccicheck review:
./fs/9p/vfs_file.c: 117: 5-8: Unneeded variable

Remove unneeded variable used to store return value.

Link: https://lkml.kernel.org/r/20211109114343.132844-1-deng.changcheng@zte.com.cn


Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>