vfs: Don't ever call openat(-1, ...) for relative paths

vfs: Don't ever call openat(-1, ...) for relative paths

This is always a bug, we should never do this. In one iteration of my
code I was doing this, which led to an invalid fallback code, which
itself lead to an infinite recursion. Make this more obvious with an
assert.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Tue Nov 12 15:13:03 UTC 2024 on atb-devel-224