arm64/gcs: release GCS on failed fork

copy_thread() can map a CLONE_VM GCS and still return 0. A later
copy_process() failure calls exit_thread(), but arm64 did not
implement it, so the VMA stayed in the parent's mm. Wire
HAVE_EXIT_THREAD to gcs_free().

Kernel threads created with CLONE_VM (io_uring and vhost workers)
inherit the parent's gcs_base/gcs_size from arch_dup_task_struct()
without allocating their own GCS. When such a thread exits,
exit_mm() -> mm_release() -> deactivate_mm() -> gcs_free() unmaps the
parent's GCS. With the new exit_thread(), a failed copy_process() for
one of these threads would do the same. Clear these fields in
copy_thread().

Fixes: 506496bcbb42 ("arm64/gcs: Ensure that new threads have a GCS")
Cc: stable@vger.kernel.org
Cc: Mark Brown <broonie@kernel.org>
Assisted-by: LLM
Signed-off-by: Iván Ezequiel Rodriguez <ivanrwcm25@gmail.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>