drm/xe/oa: Fix potential UAF in xe_oa_add_config_ioctl()

drm/xe/oa: Fix potential UAF in xe_oa_add_config_ioctl()

In xe_oa_add_config_ioctl(), we accessed oa_config->id after dropping
metrics_lock. Since this lock protects the lifetime of oa_config, an
attacker could guess the id and call xe_oa_remove_config_ioctl() with
perfect timing, freeing oa_config before we dereference it, leading to
a potential use-after-free.

Fix this by caching the id in a local variable while holding the lock.

v2: (Matt A)
- Dropped mutex_unlock(&oa->metrics_lock) ordering change from
  xe_oa_remove_config_ioctl()

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6614
Fixes: cdf02fe1a94a7 ("drm/xe/oa/uapi: Add/remove OA config perf ops")
Cc: <stable@vger.kernel.org> # v6.11+
Suggested-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Sanjay Yadav <sanjay.kumar.yadav@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patch.msgid.link/20251118114859.3379952-2-sanjay.kumar.yadav@intel.com