libbpf: parse maps sections of varying size

This library previously assumed a fixed-size map options structure.
Any new options were ignored.  In order to allow the options structure
to grow and to support parsing older programs, this patch updates
the maps section parsing to handle varying sizes.

Object files with maps sections smaller than expected will have the new
fields initialized to zero.  Object files which have larger than expected
maps sections will be rejected unless all of the unrecognized data is zero.

This change still assumes that each map definition in the maps section
is the same size.

Signed-off-by: Craig Gallek <kraig@google.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>