wifi: brcmfmac: pcie: handle randbuf allocation failure

The kzalloc() in brcmf_pcie_download_fw_nvram() will return null
if the physical memory has run out. As a result, if we use
get_random_bytes() to generate random bytes in the randbuf, the
null pointer dereference bug will happen.

In order to prevent allocation failure, this patch adds a separate
function using buffer on kernel stack to generate random bytes in
the randbuf, which could prevent the kernel stack from overflow.

Fixes: 91918ce8 ("wifi: brcmfmac: pcie: Provide a buffer of random bytes to the device")
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240306140437.18177-1-duoming@zju.edu.cn