drm/msm/gem: fix error handling in msm_ioctl_gem_info_get_metadata()

msm_ioctl_gem_info_get_metadata() always returns 0 regardless of
errors. When copy_to_user() fails or the user buffer is too small,
the error code stored in ret is ignored because the function
unconditionally returns 0. This causes userspace to believe the
ioctl succeeded when it did not.

Additionally, kmemdup() can return NULL on allocation failure, but
the return value is not checked. This leads to a NULL pointer
dereference in the subsequent copy_to_user() call.

Add the missing NULL check for kmemdup() and return ret instead of 0.

Note that the SET counterpart (msm_ioctl_gem_info_set_metadata)
correctly returns ret.

Fixes: 9902cb999e4e ("drm/msm/gem: Add metadata")
Cc: stable@vger.kernel.org
Signed-off-by: Yasuaki Torimaru <yasuakitorimaru@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/714478/
Message-ID: <20260325114635.383241-1-yasuakitorimaru@gmail.com>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>