MAINTAINERS, xsk: Update AF_XDP section after moves/adds

Update MAINTAINERS to correctly mirror the current AF_XDP socket file
layout. Also, add the AF_XDP files of libbpf.

rfc->v1: Sorted file entries. (Joe)

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: Joe Perches <joe@perches.com>
Link: https://lore.kernel.org/bpf/20200520192103.355233-16-bjorn.topel@gmail.com