9p: Remove unnecessary variable for old fids while walking from d_parent

Remove the ofid variable that's local to the conditional block in favor
of the old_fid variable that's local to the entire function.

Link: https://lkml.kernel.org/r/20220527000003.355812-5-tyhicks@linux.microsoft.com
Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>