9p: Make the path walk logic more clear about when cloning is required

Cloning during a path component walk is only needed when the old fid
used for the walk operation is the root fid. Make that clear by
comparing the two fids rather than using an additional variable.

Link: https://lkml.kernel.org/r/20220527000003.355812-4-tyhicks@linux.microsoft.com
Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>