compat: Fix build issue on RHEL 7.7.

compat: Fix build issue on RHEL 7.7.

RHEL 7.2 introduced a KABI fixup in struct sk_buff for the name
change of  l4_rxhash to l4_hash.  Then patch
9ba57fc7cccc ("datapath: Add hash info to upcall") introduced a
compile error by using l4_hash and not fixing up the HAVE_L4_RXHASH
configuration flag.

Remove all references to HAVE_L4_RXHASH and always use l4_hash to
resolve the issue.  This will break compilation on RHEL 7.0 and
RHEL 7.1 but dropping support for these older kernels shouldn't be
a problem.

Fixes: 9ba57fc7cccc ("datapath: Add hash info to upcall")
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>