linprocfs: find cwd and root handling

The code would incorrectly use curthread instead of the target proc to
resolve vnodes.

Fixes: 8d03b99b9dafe928 ("fd: move vnodes out of filedesc into a dedicated structure")
PR: 258729
Noted by: Damjan Jovanovic <damjan.jov@gmail.com>

(cherry picked from commit 69ab52838655912cf89699e0b2d21d244d3b9b27)