firmware_loader: Use security_post_load_data()

firmware_loader: Use security_post_load_data()

Now that security_post_load_data() is wired up, use it instead
of the NULL file argument style of security_post_read_file(),
and update the security_kernel_load_data() call to indicate that a
security_kernel_post_load_data() call is expected.

Wire up the IMA check to match earlier logic. Perhaps a generalized
change to ima_post_load_data() might look something like this:

    return process_buffer_measurement(buf, size,
                                      kernel_load_data_id_str(load_id),
                                      read_idmap[load_id] ?: FILE_CHECK,
                                      0, NULL);

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Link: https://lore.kernel.org/r/20201002173828.2099543-10-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>