accel/rocket: fix NULL dereference and integer overflow in rocket_job_push()

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

accel/rocket: fix NULL dereference and integer overflow in rocket_job_push()

rocket_job_push() allocates a temporary array to hold all input and
output GEM object pointers:

bos = kvmalloc_array(job->in_bo_count + job->out_bo_count,
sizeof(void *), GFP_KERNEL);
memcpy(bos, job->in_bos, job->in_bo_count * sizeof(void *));
memcpy(&bos[job->in_bo_count], job->out_bos, ...);

Two bugs exist:

1. Missing NULL check: if kvmalloc_array() fails, bos is NULL and
the subsequent memcpy() dereferences it, causing a kernel NULL
pointer dereference.

2. Integer overflow: in_bo_count and out_bo_count are both u32, set
directly from userspace-supplied in_bo_handle_count and
out_bo_handle_count with no prior validation. Their sum is computed
in u32 arithmetic and can wrap to a smaller value, causing the
allocation count passed to kvmalloc_array() to be smaller than
intended. Subsequent uses still operate on the original counts when
copying and locking objects, which may lead to out-of-bounds accesses
on the temporary array.

Fix by using check_add_overflow() to detect count overflow before the
allocation, and adding a NULL check on the allocation result.