bpf: Add getter and setter for SO_REUSEPORT through bpf_{g,s}etsockopt

Augment the current set of options that are accessible via
bpf_{g,s}etsockopt to also support SO_REUSEPORT.

Signed-off-by: Manu Bretelle <chantra@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210310182305.1910312-1-chantra@fb.com