accel/ethosu: fix OOB write in ethosu_gem_cmdstream_copy_and_validate()

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

accel/ethosu: fix OOB write in ethosu_gem_cmdstream_copy_and_validate()

The command stream parsing loop increments the index variable a second
time when a 64-bit command word is encountered (bit 14 set), but does
not re-check the loop bound before writing the second word:

for (i = 0; i < size / 4; i++) {
bocmds[i] = cmds[0];
if (cmd & 0x4000) {
i++;
bocmds[i] = cmds[1]; /* unchecked */
}
}

The buffer bocmds is backed by a DMA allocation of exactly size bytes
from drm_gem_dma_create(ddev, size), giving valid indices [0, size/4-1].

When i == size/4 - 1 on entry to an iteration and bit 14 of cmds[0] is
set, bocmds[size/4-1] is written in bounds, i is then incremented to
size/4, and bocmds[size/4] writes four bytes past the end of the
allocation.

Userspace controls both the buffer contents and the size argument via
the ioctl, making this a userspace-triggerable heap out-of-bounds write.

Fix by checking the incremented index against the buffer bound before
the second write and returning -EINVAL if the buffer is too small to
contain the extended command.