avformat/udp: increase the default buffer size of a receiving socket to 384K

avformat/udp: increase the default buffer size of a receiving socket to 384K

It is a common mistake that people only increase fifo_size when they experience
drops, unfortunately this does not help for higher bitrate (> 100 Mbps) streams
when the reader thread simply might not receive the packets in time (especially
under high CPU load) if the default 64 KB of kernel buffer size is used.

New default is determined so that common linux systems can set this buffer size
without tuning kernel parameters.

Signed-off-by: Marton Balint <cus@passwd.hu>