firmware_loader: EFI firmware loader must handle pre-allocated buffer

commit 4fb60b15 upstream.

The EFI platform firmware fallback would clobber any pre-allocated
buffers. Instead, correctly refuse to reallocate when too small (as
already done in the sysfs fallback), or perform allocation normally
when needed.

Fixes: e4c2c0ff

 ("firmware: Add new platform fallback mechanism and firmware_request_platform()")
Cc: stable@vger.kernel.org
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200724213640.389191-4-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>