Remove special case handling for getfhat(fd, NULL, handle).

There is no reason for it to behave differently from openat(fd, NULL).
Also the handling did not worked because the substituted path was from
the system address space, causing EFAULT.

Submitted by: Jack Halford <jack@gandi.net>
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D18501