compat: Fix compile warning.

compat: Fix compile warning.

In ../compat/nf_conntrack_reasm.c nf_frags_cache_name is declared
if OVS_NF_DEFRAG6_BACKPORT is defined.  However, later in the patch
it is only used if HAVE_INET_FRAGS_WITH_FRAGS_WORK is defined and
HAVE_INET_FRAGS_RND is not defined.  This will cause a compile warning
about unused variables.

Fix it up by using the same defines that enable its use to decide
if it should be declared and avoid the compiler warning.

Fixes: 4a90b277baca ("compat: Fixup ipv6 fragmentation on 4.9.135+ kernels")
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Acked-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>