kdbus: use -EMSGSIZE if userspace gives us a buffer that is too small to use

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
portal: make the recv ioctl work

Hopefully, not tested yet.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
portal_test: read a message from the ioctl

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