linux: Implement setfsuid(2) and setfsgid(2) as no-ops

These system calls exist to decouple the Linux filesystem credentials
from the effective credentials, avoiding signal exposure during
privilege transitions.
The signal permission model that motivated this was revised
in Linux 2.0, making these syscalls obsolete for new applications.

Implement both syscalls as no-ops that return the current effective
UID/GID as the previous filesystem UID/GID.
Linux returns the previous filesystem UID/GID for these syscalls
with no error indication.

Same for the equivalent setfsuid16() & setfsgid16() system calls.

Signed-off-by: Ricardo Branco <rbranco@suse.de>
PR: 294879
Reviewed by: kib, pouria
Pull-Request: https://github.com/freebsd/freebsd-src/pull/2175