selftest/bpf/benchs: Add benchmark for sockmap usage

Add TCP+sockmap-based benchmark.
Since sockmap's own update and delete operations are generally less
critical, the performance of the fast forwarding framework built upon
it is the key aspect.

Also with cgset/cgexec, we can observe the behavior of sockmap under
memory pressure.

The benchmark can be run with:
'''
./bench sockmap -c 2 -p 1 -a --rx-verdict-ingress
'''

In the future, we plan to move socket_helpers.h out of the prog_tests
directory to make it accessible for the benchmark. This will enable
better support for various socket types.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://lore.kernel.org/r/20250407142234.47591-5-jiayuan.chen@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>