drm/tests: shmem: Hold reservation lock around madvise

drm/tests: shmem: Hold reservation lock around madvise

Acquire and release the GEM object's reservation lock around calls
to the object's madvide operation. The tests use
drm_gem_shmem_madvise_locked(), which led to errors such as show below.

[   58.339389] WARNING: CPU: 1 PID: 1352 at drivers/gpu/drm/drm_gem_shmem_helper.c:499 drm_gem_shmem_madvise_locked+0xde/0x140

Only export the new helper drm_gem_shmem_madvise() for Kunit tests.
This is not an interface for regular drivers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 954907f7147d ("drm/shmem-helper: Refactor locked/unlocked functions")
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.16+
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patch.msgid.link/20251212160317.287409-5-tzimmermann@suse.de