[starnix] Ignore errors during unmap cleanup

If update_after_unmap fails to handle vmsplice unmapping or zeroing
private anonymous memory, it would return an error and abort the
update to Starnix's internal state.mappings.

However, the Zircon VMAR unmap (or overwrite) had already succeeded.
This left Starnix and Zircon mappings out of sync, leading to panics
in get_stats (e.g., when reading /proc/pid/status).

We now log these errors and proceed with removing the mappings from
Starnix bookkeeping, ensuring they stay in sync.

Test: fx test starnix_core_tests
Fixed: 526731438
Change-Id: I7f1cfdbadbf3a21125f46ea51b50841b9fcb0e1b
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1704574
Fuchsia-Auto-Submit: Guocheng Wei <guochengwei@google.com>
Commit-Queue: Guocheng Wei <guochengwei@google.com>
Reviewed-by: Maria Glukhova <mariagl@google.com>