firmware: improve LSM/IMA security behaviour

The firmware loader queries if LSM/IMA permits it to load firmware
via the sysfs fallback. Unfortunately, the code does the opposite:
it expressly permits sysfs fw loading if security_kernel_load_data(
LOADING_FIRMWARE) returns -EACCES. This happens because a
zero-on-success return value is cast to a bool that's true on success.

Fix the return value handling so we get the correct behaviour.

Fixes: 6e852651f28e ("firmware: add call to LSM hook before firmware sysfs fallback")
Cc: Stable <[e-mail]>
Cc: Mimi Zohar <[e-mail]>
Cc: Kees Cook <[e-mail]>
To: Luis Chamberlain <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Cc: [e-mail]
Signed-off-by: Sven Van Asbroeck <[e-mail]>
Reviewed-by: Mimi Zohar <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/firmware_loader/fallback.c

···
 	/* Also permit LSMs and IMA to fail firmware sysfs fallback */
 	ret = security_kernel_load_data(LOADING_FIRMWARE);
 	if (ret < 0)
-		return ret;
+		return false;
 
 	return fw_force_sysfs_fallback(opt_flags);
 }