arm64/gcs: Fix error handling in arch_set_shadow_stack_status()

In the Linux kernel, the following vulnerability has been resolved:

arm64/gcs: Fix error handling in arch_set_shadow_stack_status()

alloc_gcs() returns an error-encoded pointer on failure, which comes

from do_mmap(), not NULL.

The current NULL check fails to detect errors, which could lead to using

an invalid GCS address.

Use IS_ERR_VALUE() to properly detect errors, consistent with the

check in gcs_alloc_thread_stack().