Fixed returning an error condition to ioctl(2) (#641)

Fixed returning an error condition to ioctl(2) (#641)

When returning a negative error code by ->ioctl() to the high level
interface, no error is propagated to the low level, and the reply
message to the kernel is shown as successful.

A negative result is however returned to kernel, so the kernel can
detect the bad condition, but this appears to not be the case since
kernel 5.15.

The proposed fix is more in line with the usual processing of errors
in fuse, taking into account that ioctl(2) always returns a non-negative
value in the absence of errors.

Co-authored-by: Jean-Pierre André <jpandre@users.sourceforge.net>