MINOR: applet: adjust the allocation criticity based on the requested buffer

MINOR: applet: adjust the allocation criticity based on the requested buffer

When we want to allocate an in buffer, it's in order to pass data to
the applet, that will consume it, so it must be seen as the same as
a send() from the higher level, i.e. MUX_TX. And for the outbuf, it's
a stream endpoint returning data, i.e. DB_SE_RX.