compat: Fix up changes to inet frags in 5.1+

compat: Fix up changes to inet frags in 5.1+

Since Linux kernel release 5.1 the fragments field of the inet_frag_queue
structure is removed and now only the rb_fragments structure with an
rb_node pointer is used for both ipv4 and ipv6.  In addition, the
atomic_sub and atomic_add functions are replaced with their
equivalent long counterparts.

Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>