firmware: Add request_partial_firmware_into_buf()

Add request_partial_firmware_into_buf() to allow for portions of a
firmware file to be read into a buffer. This is needed when large firmware
must be loaded in portions from a file on memory constrained systems.

Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201002173828.2099543-16-keescook@chromium.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>