Changes

gsmtap_util: Increase source_fd's SO_SNDBUF to 4MB

Using bpftrace we spotted osmo-bts sporadically blocking on send()
of gsmtap_log for 2-4s, most probably due to the UDP send buffer being
full. The wmem_default of ~292KB is indeed a bit tight, and for instance
upstream Linux got its wmem_max to 4MB in april 2025 since it's becoming
more usual that sockets need to increase this value.

Hence, increase the SO_SNDBUF to 4MB to get some extra room and avoid
blocks (or drops if socket is later on set non-blocking) and be able to
accomodate to logging spikes better.

The kernel will take care of lowering the size set to the configured
net.core.wmem_max if needed.

Related: OS#6794
Change-Id: I5033a018dfc748b309600102b8a9ade0df014615