Changes

gsmtap_util: Set sink_fd RCVBUF and SNDBUF to minimum

We never transmit anything over the socket, nor are interested in
reading from it. We are only interested in having the socket open to
avoid ICMP errors.
Hence, set the buffers to the minimum to save some memory which we won't
ever need/use.

Change-Id: Icfbc24ff679898496666d224d49e1ff06ddffda8