drm/i915: Fix CFI violation with show_dynamic_id()

When an attribute group is created with sysfs_create_group(), the
->sysfs_ops() callback is set to kobj_sysfs_ops, which sets the ->show()
callback to kobj_attr_show(). kobj_attr_show() uses container_of() to
get the ->show() callback from the attribute it was passed, meaning the
->show() callback needs to be the same type as the ->show() callback in
'struct kobj_attribute'.

However, show_dynamic_id() has the type of the ->show() callback in
'struct device_attribute', which causes a CFI violation when opening the
'id' sysfs node under drm/card0/metrics. This happens to work because
the layout of 'struct kobj_attribute' and 'struct device_attribute' are
the same, so the container_of() cast happens to allow the ->show()
callback to still work.

Change the type of show_dynamic_id() to match the ->show() callback in
'struct kobj_attributes' and update the type of sysfs_metric_id to
match, which resolves the CFI violation.

Fixes: f89823c2 ("drm/i915/perf: Implement I915_PERF_ADD/REMOVE_CONFIG interface")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220513075136.1027007-1-tvrtko.ursulin@linux.intel.com