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

MAINTAINERS

@@ -18443,8 +18443,12 @@ R:	Jonathan Lemon <jonathan.lemon@gmail.com>
 L:	netdev@vger.kernel.org
 L:	bpf@vger.kernel.org
 S:	Maintained
-F:	kernel/bpf/xskmap.c
+F:	include/net/xdp_sock*
+F:	include/net/xsk_buffer_pool.h
+F:	include/uapi/linux/if_xdp.h
 F:	net/xdp/
+F:	samples/bpf/xdpsock*
+F:	tools/lib/bpf/xsk*
 
 XEN BLOCK SUBSYSTEM
 M:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>