os: fdtable: restore errno if optional locking ioctl fails

The `zvfs_finalize_typed_fd()` function notifies some backends
via `ioctl()` with `ZFD_IOCTL_SET_LOCK`. However, support for
this method and functionality is optional.

In backends that do not support locking, this benign failure can
set `errno` to 95 (`EOPNOTSUPP`) in many circumstances where a
change in `errno` (indicating some kind of failure) is not
appropriate.

Prevent errno poisoning by backing-up and restoring `errno`.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>