9p: Track the root fid with its own variable during lookups

Improve readability by using a new variable when dealing with the root
fid. The root fid requires special handling in comparison to other fids
used during fid lookup.

Link: https://lkml.kernel.org/r/20220527000003.355812-3-tyhicks@linux.microsoft.com
Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>