ACPI: APEI: GHES: Extract helper functions for error status handling

ACPI: APEI: GHES: Extract helper functions for error status handling

Refactors the GHES driver by extracting common functionality into
reusable helper functions:

1. ghes_has_active_errors() - Checks if any error sources in a given list
   have active errors
2. ghes_map_error_status() - Maps error status address to virtual address
3. ghes_unmap_error_status() - Unmaps error status virtual address
4. Use `guard(rcu)()` instead of explicit `rcu_read_lock()`/`rcu_read_unlock()`.

These helpers eliminate code duplication in the NMI path and prepare for
similar usage in the SEA path in a subsequent patch.

No functional change intended.

Tested-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Link: https://patch.msgid.link/20260112032239.30023-3-xueshuai@linux.alibaba.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>