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

In the Linux kernel, the following vulnerability has been resolved:

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.

Patchwork: https://patchwork.freedesktop.org/patch/714478/